added equals method to expr
added equals method to expr

>>> x**2-1
x**2 - 1
>>> _.equals((x+1)*(x-1))
True
>>> (cos(x)**2+sin(x)**2).equals(1)
True
>>> (a*cos(x)**2+a*sin(x)**2).equals(a)
True

abhiii5459/sympy,kmacinnis/sympy,garvitr/sympy,MridulS/sympy,skidzo/sympy,Shaswat27/sympy,wanglongqi/sympy,shikil/sympy,jbbskinny/sympy,hargup/sympy,meghana1995/sympy,hrashk/sympy,Arafatk/sympy,shipci/sympy,Gadal/sympy,Davidjohnwilson/sympy,Gadal/sympy,wyom/sympy,hargup/sympy,ahhda/sympy,atreyv/sympy,sampadsaha5/sympy,kaichogami/sympy,jbbskinny/sympy,sunny94/temp,kaichogami/sympy,ChristinaZografou/sympy,MridulS/sympy,atreyv/sympy,sahmed95/sympy,saurabhjn76/sympy,Vishluck/sympy,postvakje/sympy,yukoba/sympy,maniteja123/sympy,oliverlee/sympy,Titan-C/sympy,MechCoder/sympy,rahuldan/sympy,asm666/sympy,debugger22/sympy,garvitr/sympy,cswiercz/sympy,sahilshekhawat/sympy,shikil/sympy,Mitchkoens/sympy,ahhda/sympy,cswiercz/sympy,maniteja123/sympy,wanglongqi/sympy,hargup/sympy,jbbskinny/sympy,shipci/sympy,madan96/sympy,ChristinaZografou/sympy,abloomston/sympy,ga7g08/sympy,rahuldan/sympy,abloomston/sympy,drufat/sympy,drufat/sympy,hrashk/sympy,MechCoder/sympy,sahilshekhawat/sympy,mafiya69/sympy,AunShiLord/sympy,jerli/sympy,ga7g08/sympy,jamesblunt/sympy,Titan-C/sympy,aktech/sympy,chaffra/sympy,iamutkarshtiwari/sympy,Arafatk/sympy,Curious72/sympy,kumarkrishna/sympy,kaushik94/sympy,emon10005/sympy,postvakje/sympy,AkademieOlympia/sympy,diofant/diofant,Designist/sympy,jaimahajan1997/sympy,beni55/sympy,pbrady/sympy,Sumith1896/sympy,lidavidm/sympy,atsao72/sympy,wyom/sympy,wyom/sympy,pandeyadarsh/sympy,sampadsaha5/sympy,cccfran/sympy,kmacinnis/sympy,kaichogami/sympy,AunShiLord/sympy,toolforger/sympy,souravsingh/sympy,iamutkarshtiwari/sympy,bukzor/sympy,Shaswat27/sympy,Designist/sympy,dqnykamp/sympy,maniteja123/sympy,jerli/sympy,jaimahajan1997/sympy,sahmed95/sympy,chaffra/sympy,srjoglekar246/sympy,flacjacket/sympy,emon10005/sympy,atsao72/sympy,VaibhavAgarwalVA/sympy,yukoba/sympy,skidzo/sympy,madan96/sympy,ga7g08/sympy,abloomston/sympy,cccfran/sympy,oliverlee/sympy,moble/sympy,sampadsaha5/sympy,mcdaniel67/sympy,amitjamadagni/sympy,jamesblunt/sympy,Vishluck/sympy,vipulroxx/sympy,Mitchkoens/sympy,yashsharan/sympy,liangjiaxing/sympy,madan96/sympy,saurabhjn76/sympy,sunny94/temp,kevalds51/sympy,hrashk/sympy,moble/sympy,AkademieOlympia/sympy,rahuldan/sympy,beni55/sympy,lindsayad/sympy,kmacinnis/sympy,skidzo/sympy,pbrady/sympy,aktech/sympy,farhaanbukhsh/sympy,skirpichev/omg,Titan-C/sympy,sahilshekhawat/sympy,cswiercz/sympy,Vishluck/sympy,amitjamadagni/sympy,beni55/sympy,jaimahajan1997/sympy,saurabhjn76/sympy,moble/sympy,farhaanbukhsh/sympy,debugger22/sympy,pandeyadarsh/sympy,meghana1995/sympy,shipci/sympy,AunShiLord/sympy,souravsingh/sympy,drufat/sympy,mcdaniel67/sympy,abhiii5459/sympy,debugger22/sympy,pbrady/sympy,grevutiu-gabriel/sympy,lindsayad/sympy,bukzor/sympy,atreyv/sympy,Curious72/sympy,cccfran/sympy,souravsingh/sympy,sunny94/temp,kevalds51/sympy,atsao72/sympy,VaibhavAgarwalVA/sympy,asm666/sympy,jerli/sympy,kevalds51/sympy,dqnykamp/sympy,MridulS/sympy,Gadal/sympy,lidavidm/sympy,VaibhavAgarwalVA/sympy,Davidjohnwilson/sympy,farhaanbukhsh/sympy,liangjiaxing/sympy,mafiya69/sympy,lindsayad/sympy,chaffra/sympy,ChristinaZografou/sympy,kaushik94/sympy,meghana1995/sympy,abhiii5459/sympy,Davidjohnwilson/sympy,Mitchkoens/sympy,yukoba/sympy,emon10005/sympy,liangjiaxing/sympy,aktech/sympy,wanglongqi/sympy,shikil/sympy,lidavidm/sympy,garvitr/sympy,AkademieOlympia/sympy,pandeyadarsh/sympy,iamutkarshtiwari/sympy,vipulroxx/sympy,MechCoder/sympy,kumarkrishna/sympy,sahmed95/sympy,ahhda/sympy,Sumith1896/sympy,yashsharan/sympy,dqnykamp/sympy,Arafatk/sympy,toolforger/sympy,mcdaniel67/sympy,bukzor/sympy,postvakje/sympy,kaushik94/sympy,mafiya69/sympy,yashsharan/sympy,grevutiu-gabriel/sympy,grevutiu-gabriel/sympy,vipulroxx/sympy,jamesblunt/sympy,oliverlee/sympy,kumarkrishna/sympy,Sumith1896/sympy,Curious72/sympy,Designist/sympy,Shaswat27/sympy,asm666/sympy,toolforger/sympy