Adds Record::find() method

As requested by @mvdkleijn in issue #461, this is a pull request for my (unfinished) work on the Record class.

Note the following:

* This is a work in progress and **unfinished**!
* It uses the PHP function get_called_class() and therefore requires **PHP 5.3.0 or higher**.
* <del>Currently based, and only tested on MySQL and might contain **MySQL-specific** parts</del>