Fix bugs in sqlmap formula and reorganize options.

Fix a bug causing `psycopg` to fail to build because `pg_config` is not found.

Replace `--with-db-modules` 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 the needed one is installed). So, when sqlmap 1.0 is out, it will probably be safe to remove MySQL as a dependency.

Tested both stable and HEAD on OS X 10.7 and 10.9. Note that, if you use Xcode 5.1, currently you need to instruct `clang` to tolerate invalid compiler options, e.g.:

    ARCHFLAGS=-Wno-error=unused-command-line-argument-hard-error-in-future brew install sqlmap --with-mysql