Sorl-thumbnail-async

Since `sorl-thumbnail` already supports Python 3, this package also supports it from from version 0.6.0

After [shanx 's improvement](https://github.com/neokya/sorl-thumbnail-async/pull/1), I am temped to make first ever release.
1. Allow user to define THUMBNAIL_OPTIONS_DICT in own settings file which is properly used
2. Define a get_thumbnail() call that takes the alias name and returns thumbnail. So that in python code sorl-thumbnail-async can be used easily as well.

Thanks a lot shanx :)