Activate clicked object if group selection is cleared

If group selection is cleared and pointer clicked other object 
=> activate this object.

See issue #506
Don't set activeObject if clicked on activeGroup

canvas._activeGroup should be null if clicked on activeGroup. 
Only set activeObject if target !== activeGroup.
Merge pull request #534 from Kienz/patch-1

Activate clicked object if group selection is cleared
Merge remote-tracking branch 'origins/master' into sketch