Introduced DEFAULT setting for size definition

Created new class DefaultThumbnail to represent Default Thumbnail.
Gallery will return DefaultThumbnail if SIZES definition contains
DEFAULT key.
Deleted unused size property on DefaultThumbnail
Refactor default thumbnail

DefaultThumbnail is gone. We will use Thumbnail with extra params
``default`` during __init__ method. This will be much more elegant
than defining new class
Merge branch 'master' into default-picture

Also updated tests

Conflicts:
	thumbnails/tests/settings.py
Revert "Refactor default thumbnail"

This reverts commit b5c0c365ef3d8be2e249a78ae8804ad853f34af4.

Conflicts:
	thumbnails/tests/test_fields.py
Changed DEFAULT key in settings to FALLBACK_IMAGE_URL
Tweak the code
Refactor images.py and files.py

1. Renamed DefaultThumbnail -> FallbackImage
2. Renamed variable FALLBACK_IMAGE_URL -> fallback_image_url
Use isinstance to test for class
Merge pull request #54 from SeiryuZ/default-picture

Introduced DEFAULT setting for size definition