Added a number of improvements, fixed bugs and improved docs

Heya! Thanks for publishing sorl-thumbnail-async.

I've added a number of improvements:
 - Allow user to define THUMBNAIL_OPTIONS_DICT in own settings file which is properly used
 - 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.

Furthermore I fixed a number of small issues and extended the documentation.

Cheers,
Remco