Jaxb support + AT-9 feed pagination is configurable

* added Jaxb support for xml serialization (JFeed java model)
* added Jackson support for Json serialization (for java)
* AT-9 feed pagination is configurable
* HTTP optimizations (ETag support, caching headers, ...)
* embedded mongo for tests
* JdbcFeedStore tests on H2 in memory DB
* Play server (demo code) + FeedSupport trait