Metadata persistence in HDFS and MySQL

Partially implements the issue https://github.com/telefonicaid/fiware-connectors/issues/41

100% tests passed
Manual tests passed
Because I'm now using the `@Test` annotation. This annotation also allows me to clearly define which methods inside the class are real tests. If you observe `testGetContextResponses`, it has been split into 6 different sub-tests which are not real tests but "convenience functions". I tried to made them private, but Maven always tried to execute them and warn about they were not public! Thus the solution was to annotate which mehods were real tests.