Extracted new method for registering resource on the fly.

Refactored Eve's configuration handling and created Eve.register_resource()
method. If there exists given resource in the config, its settings is
overwritten by calling this method.

This may be base method for all ODM/ORM adapters which create schema out of
model and need to add it into Eve's config. #126
Added tests for Eve.register_resource().

#126.