Add support for generic signals
Add support for generic signals

This is achieved by adding a keyword (signals) to register. This accepts
a list-like item containing signals to connect to. Some special-casing
was added to handle pre_delete. The default is still to just connect to post_save.

MikeAmy/django-reversion,IanLee1521/django-reversion,blag/django-reversion,blag/django-reversion,pydanny/django-reversion,ixc/django-reversion,Beauhurst/django-reversion,adonm/django-reversion,etianen/django-reversion,lutoma/django-reversion,matllubos/django-reversion,adonm/django-reversion,mkebri/django-reversion,lutoma/django-reversion,talpor/django-reversion,talpor/django-reversion,etianen/django-reversion,Beauhurst/django-reversion,ixc/django-reversion,mkebri/django-reversion,pydanny/django-reversion,IanLee1521/django-reversion,MikeAmy/django-reversion,matllubos/django-reversion