Full SQLAlchemy intégration

Hello,

When trying to use rdflib-sqlalchemy for I project, I encountered various problems with string encoding and quote escaping. From your latest activity it seems that you encountered some of them too.

When checking the code to see if an easy correction was possible , I rapidly saw that the potential of SQLAlchemy was barely used, and that a significant part of the code was trying to tackle problems already solved successfully by SQLAlchemy.

In my port, when possible, I have tried not to changed the interface and behaviour of the code. However the following points may differ from the original code:

-  Exceptions raised when adding a triple are no longer swallowed
-  All columns names are now lower case. this has been necessary to handle the different behaviour about cases sensitivity in database schema between engines. This should have no impact on existing database for postgres or sqlite, but I don't know if this is the case for other engines like mysql.
-  I did not try to tackle the REGEXP portability problem. It is not covered natively by SQLAlchemy and some custom code has to be written to ensure a consistent behaviour on all engines.  Please note that this problem is not solved either in the original code.

All unit tests pass for all combinations of python 2.7, python 3.3, postgres, mysql(db) and sqlite. 

Regards,

Yves-Marie Haussonne