Added fixtures for the sentence model.

Along with everything linked to it.
Created a simple test for adding sentence.
Fixed $useTable attribute of the CurrentUser model

This attribute can only take false or a string as value.
Fixed the include path of current_user.php.

The include path doesn't work in tests context.
Include Sanitize in the Sentence model.

So that it also work in tests context.
Removed bogus model relationship.

Sentences and Tags are already linked with a HABTM relation.
Fixed parent constructor call parameters.

So that the tests of this model use the test database instead of
the real one.