Add PostgreSQL database driver

This is regarding [bug 3849](https://bugs.alliedmods.net/show_bug.cgi?id=3849).
Adds a "pgsql" driver to the dbi system.

See https://forums.alliedmods.net/showthread.php?t=236415

The AMBuild changes are very likely wrong. I don't know how to use it ;)

Here are some instructions on how to create static libs of the libpq out of the [source](http://ftp.postgresql.org/pub/source/v9.3.4/postgresql-9.3.4.tar.gz)

Get the latest postgresql source.
extract it.

=============
Windows:

In a VS-commandline, navigate to src/tools/msvc and run
  perl mkvcbuild.pl
to generate the VS solution in the root.

1. Open the pgsql.sln in VS2010 
2. change the configuration type of the libpq project from "Dynamic library (.dll)" to "Static library (.lib)"
3. under C/C++ -> codegeneration change runtimelibrary from "Multithreaded-Debug-DLL (/MDd)" to "Multithreaded-Debug (/MTd)" for the libpq and libpgport projects
4. compile the libpq project
5. the static .lib will be in Release/libpq/libpq.lib and Release/libpgport/libpgport.lib

==================
Linux:

in the extracted postgresql source run
  ./configure LDFLAGS='-static -m32' CFLAGS='-m32' --prefix=`pwd`/build
You might need to add --without-readline. This is not needed for the libpq library.
  cd src/interfaces/libpq
  make install
  rm libpq.a
  make install enable_shared=no
The library will be in the build/lib directory.

About that double linking to get a good static library: http://www.postgresql.org/message-id/1902207738.20070920111734@comodo.com
Whoops - I can't find my original comment, but I think when I wrote it I assumed we'd be checking the postgres source into the tree. If we're not doing that - and we probably shouldn't if it's ~100MB - this approach won't work. AMBuild commands operate within the build folder. Anything outside the build folder is considered immutable.

AMBuild can't handle external build jobs, because it doesn't understand their dependencies. I'm not sure how to work around this. Maybe AMBuild needs a way to specify precursor-step scripts that can run external builds.

So, where does that leave us... I'd originally hoped that we could just use DLLs, so that's still an option. We could add build jobs for postgres and host the static libs ourselves. We could fork postgres into GitHub with SM's files added, and check it out into the SM tree as part of the build process.

DLLs still seem like the best option to me.