Make ImageCropField widget overwritable

It's important so we can create custom widgets to ImageCropField
Merge pull request #27 from sayadn/master

Make ImageCropField widget attribute overwritable
Use ImageCroppingMixin in tests
Correct easy-thumbnails template loading example
Add Simon Meers to contributors
Improve thumbnail processor a little
Tweak thumbnail processor a little
Adjust link target for Travis CI build image
Updated Jcrop from 0.9.9 to 0.9.12 for compatibility with jQuery 1.9+, fixes #24
Added rounding of Jcrop values in javascript

Jcrop removed rounding of floats when cropping scaled images in 0.9.10, causing
easy_thumbnails to fail to resize the image.

This change restores that logic, moving it from Jcrop to image_cropping.js,
ensuring that only integers are ever stored as they were before updating Jcrop.
Update jQuery to 1.9.1
Add Richard Terry to contributors
Tweak thumbnail processor

The box argument can be either a string or a list.
Bump version for new release
Don't pin specific easy thumbnails version as requirement
Allow building docs via sphinx
Only define version string in one place
Added changelog to README
Tweak MANIFEST.in
Update travis config