Reorganize for Composer support and PSR-4 Autoload, Move back to support PHP 5.3

This PR should not affect real-world usage at all.

* I switched all of the `[]` array syntax with `array()` to make the project work with PHP 5.3.
* I reorganized the file structure to conform with the [PSR-4 Autoload Standard](http://www.php-fig.org/psr/psr-4/).
* I generated an autoloader with Composer and replaced all the include / require stuff in the main plugin file with a reference to that autoloader. 
* I added a helper function to `WordPress\ORM\BaseModel` to fetch the table prefix from `$wpdb`.
* I added a composer.json file to allow it to be published to Packagist.

Thanks for the great tool! It's saving me a lot of time and a bunch of code.