Update to spring-data-commons SNAPSHOT

Incorporated recent changes to Spring Data Commons and dependent projects that obsoleted the need to manage domain object metadata within Spring Data REST. Required updating to the latest snapshots available for spring-data-commons and spring-data-jpa. 

Additional changes include:

* Re-wrote the monolithic Controller into separate controller classes that have a more narrow focus.
* Implemented common functionality as a `HandlerMethodArgumentResolver` rather than as a helper method in a controller class.
* Re-implemented JSONP functionality as an HttpMessageConverter rather than inline within a controller class.
* Updated to Jackson 2 for all JSON handling.
* By relying on spring-data-commons, spring-data-rest now handles all supported Repository types: JPA, MongoDB, and GemFire.

Added support for MongoDB and GemFire repositories by relying on spring-data-commons to provide the metadata rather than maintaining internal metadata information that is store-specific.

Replaced Spock spec tests with JMock unit and integration tests. Started integrating Jetty 8 into the testing so MVC testing can be done against a live server.