unsavedChanges: pass a reference to the changed model

This will be useful for associating the changed attributes with a
particular model.  I am interested in having access to the model's id so
that I can batch save many models at once.
update to support patch
updated tests and readme
Merge branch '7-add-support-for-patch-method-in-sync' of github.com:lalitkapoor/backbone.trackit into lalitkapoor-7-add-support-for-patch-method-in-sync

* '7-add-support-for-patch-method-in-sync' of github.com:lalitkapoor/backbone.trackit:
  update to support patch
updated readme