Fix a bug causing psycopg to fail to build because pg_config is not found.

Replace --with-db-modules option with an option for each database driver. The
rationale is that some Python modules depend on a DBMS being installed (e.g., MySQL,
PostgreSQL), while others don't.

Add optional dependencies from MySQL and PostgreSQL. The dependency from MySQL
is needed only when --with-mysql is requested and only for the stable version of sqlmap,
which uses python-mysqldb. The HEAD version depends on pymysql, which does not
need MySQL to be built (the previous version of the formula always installed both
modules, but now only one is installed).
sqlmap: add 0.9 bottle.