Use thumbnail formats defined in settings
Update README.rst: use THUMBNAIL_MAKER_FORMATS

THUMBNAIL_MAKER_FORMATS - a dictionary with all thumb formats.
Remove unneeded import, add docstrings
Merge branch 'feature-thumbs-in-settings' of github.com:infinityxxx/django-thumbnail-maker into feature-thumbs-in-settings
Merge pull request #1 from infinityxxx/feature-thumbs-in-settings

Rewrite base configuration of application