Metadata persistence in HDFS and MySQL

Partially implements the issue https://github.com/telefonicaid/fiware-connectors/issues/41

100% tests passed
Manual tests passed
You are right we do not need the metadata stuff because this is only about table creation.

Anyway, there is a much more complex problem behind this, and it is related to the answer to the first question: in the **row** mode no table name was needed because all the files within the same HDFS folder were going to be added to the same table. Why and how was this possible? Because the external tables work by pointing to a HDFS folder and because all the tables has the same attributes. But with the **column** mode we have several files containing each one a different number of columns. Thus a unique Hive table for all the files within the same HDFS folder in not valid anymore.

I have to think in this issue, but the first obvious solution seems to be to have a subfolder per each file/entity:
* Current folder structure (only valid for the row mode, a single Hive table):
    `hdfs:///user/basuras_valencia/entity_1_file.txt`
    `hdfs:///user/basuras_valencia/entity_2_file.txt`
* New proposal (valid for both row and column mode, a Hive table per each entity, as MySQL and CKAN do):
    `hdfs:///user/basuras_valencia/entity_1/entity_1_file.txt`
    `hdfs:///user/basuras_valencia/entity_2/entity_2_file.txt`

What do you think, @fgalan ?