Implement Action#format

* Add #format
* Add #format= which also sets content-type
* Remove #content_type=
* Fix integration test
Add documentation for Action#format=
Documentation typo

:shit:
Merge pull request #18 from zlw/15_action_format

Implement Action#format
Let actions to detect accepted mime type and to return the correct format symbol. Configuration can now register mime types
Detect the asked mime type and return the corresponding format
Make Lotus::Action::Mime#format a public method
Additional example for Mime type integration tests