JSON serialization of RegisteredService into LDAP

Handles https://github.com/Jasig/cas/issues/681

I striped down the `DefaultLdapMapper` so the entire service definition is persisted into a single attribute as JSON. There are setters that allow extending the serializer, and I also modified the tests to adjust the objectclass required. Some classes had to include getters for serialization to properly work. 
What's the use of these ```readJsonTree``` methods?