Add test for config from envvar.

Added a new base class for tests that retain the current envvar settings,
so that they are unaffected by test runs. Made all tests inherit from this
base class.
Add a fk_exists validator.

Can be used to validate that the object with the primary key references
in a foreign key exists.
Add a Entry model and corresponding views.

Testing the entry also required adding a marvin.validators model,
which can hold custom validators we might need. For now that includes
a fk_exists validator, to make sure that the foreign_key provided points
to an existing object.

Docs and tests included.