Created BaseSearchResults class

Moved some logic from ElasticSearchResults into it
DB search now returns a SearchResults object

Previously, it returned a QuerySet which gave the database backend much
more API than the Elasticsearch backend has. Which could potentially
cause issues if the developer is using the DB backend locally and the
Elasticsearch backend on their production server.

Fixes #460
Created BaseSearchQuery class

Moved some logic from ElasticSearchQuery into it
Added DBSearchQuery class

This makes sure that the user is not filtering on any fields that are
not in search_fields. It also prevents the user from using any features
of QuerySet that are not in the search API
Make default methods raise NotImplementedError
Clarify how fill works with images that can't upscale.
Fixing meaningless test that would forever fail.

IE9 doesn't support conditional comments so testing "if IE 9" would always be false. Additionally this also meant no HTML tag was actually output for any other browser either! As we don't support IE8 either, testing for IE8 was also pointless.
Merge pull request #637 from kaedroho/search/api-fixes

Search: Make database backend behave like Elasticsearch backend. Fixes #460
Return None when no features detected
Fetched new translations from transifex
Run makemessages
Run compilemessages
Merge pull request #752 from kaedroho/images-refactor

Images: Remove FocalPoint class
Merge pull request #783 from kaedroho/translations-08

Translation updates for wagtail 0.8
Added Willow to install requirements
Added get_willow_image method to Image
Added new image operations
Added new filter running code

- Loads image operations using hooks
- Uses Willow

Not yet removed old code to keep commit changes as clean as possible
Added get_vary and get_vary_key to Filter

This allows a filter operation to specify which strings of data it used
inside the image to perform its calculations. This can be used to
determine whether an image has changed in such a way that requires new
renditions to be made.