Inform ValidatingMongoEventListener to use LocalValidatorFactoryBean

This is to enable hibernate validator annotations such as @Email and @Url

See gh-8
Extract annotations and remove test cases from UserRepositoryTest

Annotations are now being placed in AbstractTest as there will be more test classes that will be using it. While tests related to sites are being pulled out and will be done in its own repository class.

See gh-8
Mark sites as Transient

Information regarding user sites will be stored in its own collection.

See gh-8
Add implementation for UserSite

UserSite consists of userId, site's title, and url. User class will retrieve which sites belong to a user through userId.

Fixed gh-8
Add yml for test purpose