API: Use default Language manager for LanguageResource

This way it is possible to access to all languages and not just those that
are used on at least a translation project.
API: Complete ProjectResource and add docs

Fix bug 2951
API: Complete LanguageResource and add docs

Fix bug 2952
API: Complete TranslationProjectResource and add docs

Fix bug 2953
API: Add StoreResource and docs

Fix bug 2954
API: Add UnitResource and docs

Fix bug 2955
API: Add SuggestionResource and docs

Fix bug 2956
API: Provide statistics through /resource/pk/statistics/ like URLs

The following resources are now subclasses of StatisticsModelResource:

* LanguageResource
* ProjectResource
* StoreResource
* TranslationProjectResource

Also added new docs for this feature on all the affected resources

Added necessary methods on Project, Language and TranslationProject models

Fix bug 2957