Support other drivers

Support other drivers by defining a connection interface and letting MytermMysqlConnection implement that. This work will require that some mysqli-specifics be turned into more generic forms. Also, the FieldInfo class will have to adapt to a generic abstraction instead of relying on the MYSQLI_xxx predefined constants.

It's probably a good idea to do this later rather than sooner, when we have a better understanding of the common requirements for a driver.