API: Add StoreResource and add docs

Fix bug 2954
NEEDS WORK    API: Add more resources and change others

Added resources:

* Unit resource
* Suggestion resource

Changed resources:

* Store resource
API: Use new Resource class for providing /resource/pk/statistics/ URLs

The following are now subclasses of StatisticsModelResource:

* LanguageResource
* ProjectResource
* StoreResource
* TranslationProjectResource

Also added new docs for this feature on all the affected resources
API: Add filtering using field lookups to UnitResource
NEEDS WORK    Docs: Add additional API docs
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
NEEDS WORK    Docs: Add docs for API UserResource
UNDO    API: Add sample pootle-cli client for API

It is meant for learning purposes
UNDO    IGNORE THIS: misc comments
UNDO  not working tests with pootle-cli