Add assertAreNotNone.
Add assertAreNotNone.

This runs through the attributes provided in the attrs argument and checks
that each will not return None when accessed on the object passed in. This
makes checking objects with tons of attributes far easier.

balloob/github3.py,krxsky/github3.py,sigmavirus24/github3.py,h4ck3rm1k3/github3.py,itsmemattchung/github3.py,wbrefvem/github3.py,degustaf/github3.py,jim-minter/github3.py,agamdua/github3.py,ueg1990/github3.py,icio/github3.py,christophelec/github3.py