Fix build: ext/ext_string.h => ext/string/ext_string.h
Merge pull request #33 from tlevi/ext_string

Fix build: ext/ext_string.h => ext/string/ext_string.h
Replace the NEWOBJ macro with the newres<T> function.

This way this extension builds with HHVM's master branch.
Handle errors on the `pg_execute` function properly.

Before this patch executing `pg_execute` always returned the statement object,
even on failure. This patch fixes this so it matches the specification of this
function as defined here:

    http://php.net/manual/en/function.pg-execute.php

According to the README of this project, the `pg_execute` function should
behave in the same way as in the Zend implementation.
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.
Polishing.
tests: removed some unneeded methods from the Test class.
tests: added some documentation to the Test class.
Updated the README.md file.
tests: fixed a couple of syntax errors and set a proper exit status code.