Do a better event re-injection and fix quotes in CKAN attrValues

This PR implements the issue https://github.com/telefonicaid/fiware-connectors/issues/52 which has been shown in https://github.com/telefonicaid/fiware-connectors/issues/103 (opened) and https://github.com/telefonicaid/fiware-connectors/issues/123 (closed)

This PR also fixes the issue described at https://github.com/telefonicaid/fiware-connectors/issues/106 (opened)
Basically, the changes are:

* 3 new Exception classes have been added: `CygnusBadConfiguration`, `CygnusPersistenceError` and `CygnusRuntimeError`.
* In the backend classes, where a Exception was thrown, one of the above exception classes is now thrown; this way we can distinguish among them.
* In the OrionSink base class, where the `process` method is, we call to `persist(event)`  (which in the end calls to the specific sink persistence method) and try to catch exceptions. If nothing is catched, everything goes well. If something is catched, we decide if the Flume event must be re-injected (`Cygnus PersistenceError`) or simply logged (the other cases).

It worths mention that in the backends all the code is suceptible to launch an exception: there where a specific `CygnusPersistenceError` or `CygnusBadConfiguration` is not thrown, a general `CygnusRuntimeError` can be generated is something (whatever) goes bad.