Add database capability: SQLAlchemy / declarative.

Add database capability through SQLAlchemy. Base
(http://docs.sqlalchemy.org/en/rel_0_8/orm/extensions/declarative.html#module-sqlalchemy.ext.declarative)
and database session
(http://docs.sqlalchemy.org/en/rel_0_8/orm/session.html#what-does-the-session-do)
are setup by database_helper module in helper package.

The DevelopmentConfig class has gained a DB_URL key, that states
on which server the database lives.