Metadata persistence in HDFS and MySQL

Partially implements the issue https://github.com/telefonicaid/fiware-connectors/issues/41

100% tests passed
Manual tests passed
Instead of using the `substring(0, columnValues.length() - 1)` for removing the ",", you can selectively add it in the while loop using the it.next(). 

Example (from the CKAN branch):

```
        while (it.hasNext()) {
            String attrName = (String) it.next();
            String attrValue = attrList.get(attrName);

            attrs += "\"" + attrName + "\": \"" + attrValue + "\"";
            if (it.hasNext()) {
                attrs += ", ";
            } // if
        } // while
```