Changed API for setting/getting focal points

Previously, we used a property to do this. Heres the advantages of the
new way:
 - The old way felt a bit like it was pretending to be a database field
   when it wasn't. The new way makes it easier for the developer to
   understand that this is just a setter/getter for 4 fields and not a
   field itself.
 - Code looks nicer
 - Easier to override in subclasses
 - More like Django user model
Removed FocalPoint class

And refactored code that used it to use Rect instead
Added get_rect method to image
Added tests for Rect class
Added tests for set/get focal point methods
Removed wagtailimages/utils folder
Return None when no features detected
Merge pull request #752 from kaedroho/images-refactor

Images: Remove FocalPoint class