Bugfix in apply_markup_filter()

lxml fails if text is empty string when applying markup filter

It is necessary to this work for the API.
API: add support Pootle API using Tastypie

* Basic API capable of listing used languages
* Add exception in CaptchaMiddleware to let pass API requests
* Include Tastypie in base requirements
* Add new settings for the API
* Docs about new settings for the API
API: add initial support for listing projects and TPs
API: add initial API docs

[ci skip]
PEP8 fixes.
Add String and Number in Javascript styleguide
Formatting fixes, small reword.