API: Add more resources and change others

Added resources:

* Store resource
* Unit resource
* Suggestion resource

Changed resources:

* Language resource
* Project resource
* Translation project resource
API: Use new Resource class for providing /resource/pk/statistics/ URLs

LanguageResource, ProjectResource, StoreResource and
TranslationProjectResource are now subclasses of StatisticsModelResource
API: Add filtering using field lookups to UnitResource
UNDO    Docs: Add additional API docs

Still needs work
API: Add UserResource resource
API: Add UserObjectsOnlyAuthorization

This will allow to forbid access to User resources that belong to other users
API: Add statistics to UserResource
UNDO    Docs: Add docs for API UserResource

needs stabilising UserResource first
UNDO    API: Add sample pootle-cli client for API

It is meant for learning purposes
UNDO    IGNORE THIS: misc comments