Let's use the FindPostgreSQL.cmake file that comes bundled with CMake.

This way we don't have to look for libpq-fe.h or pq manually.
Pick the proper variable in CMake that points to PostgreSQL.
Moved the source files into the `src` directory and created the `test` dir.

I've also written the basics in order to get PHPUnit up and working.
tests: implemented some tests for the `pg_execute` function.

I've decided that it's not worth it to use PHPUnit. We're just going to use a
custom class with asserts.
tests: added some tests for the `pg_prepare` function.
tests: let the user filter the tests to be run.