Add support for MariaDB

[fmohr]

The MariaDB connector does not support explicit transactions. This is a bit annoying since the two database systems are usually considered to be substitutable, and MySQL is not (anymore) available in several Linux distributions. However, except from this, it works 100% identical to the MySQL connector. The cleanest solution would probably be to add a flag in the constructor of DatabaseConnectorMYSQL called explicit_transactions = True. A MariaDB connector could just inherit from this connector and set the flag to False at construction time.