move window to current group
move window to current group

added possibility to move a window to the current group with togroup()
method when None sent

Closes #707

qtile/qtile,cortesi/qtile,jdowner/qtile,ramnes/qtile,farebord/qtile,StephenBarnes/qtile,himaaaatti/qtile,frostidaho/qtile,kopchik/qtile,dequis/qtile,farebord/qtile,flacjacket/qtile,apinsard/qtile,rxcomm/qtile,soulchainer/qtile,soulchainer/qtile,kseistrup/qtile,StephenBarnes/qtile,flacjacket/qtile,de-vri-es/qtile,zordsdavini/qtile,zordsdavini/qtile,rxcomm/qtile,nxnfufunezn/qtile,tych0/qtile,apinsard/qtile,de-vri-es/qtile,himaaaatti/qtile,dequis/qtile,xplv/qtile,nxnfufunezn/qtile,ramnes/qtile,qtile/qtile,jdowner/qtile,tych0/qtile,kynikos/qtile,kopchik/qtile,cortesi/qtile,frostidaho/qtile,kynikos/qtile,xplv/qtile,kseistrup/qtile