MySQL persistence, unit tests, sinks refactoring, HDFS backends refactoring, README r0.2

This is a big PR covering several issues:
* MySQL persistence (issue https://github.com/telefonicaid/fiware-connectors/issues/42)
* Unit tests (issue https://github.com/telefonicaid/fiware-connectors/issues/18)
* Sinks and HDFS backends refactoring (issue https://github.com/telefonicaid/fiware-connectors/issues/34)
* New README for release 0.2