Feature psr0 loader

I added the reference PSR-0 autoloader and moved the default file to manually require to *loader.php*. Updated the Readme accordingly.

The *Braintree.php* file attempts to detect whether a suitable autoloader is operating (such as provided by composer). If not, it will load the *loader.php* file as an attempt to be backwards-compatible.

I ran unit tests and everything seems ok.