Refactor with modern tools

Hi,

This follow a discussion I had with one staff member of Pusher. I think that such a great service needs a better PHP library, as the current one does not really use any modern tools, nor does not really respect any of modern PHP convetnions (PSR-0/1/2).

This refactor is powered by Guzzle library (which was used by Amazon to power their new AWS SDK) and it provides a lot of benefits over what is done currently.

First, it's far more flexible as we have access to an event manager. Then, it has a built-in parameter validation/filter. It's also easier to maintain thanks to the service description, which also acts as a documentation.

Of course, this is absolutely not backward-compatbile with the current library, so this one would be 3.0.0. This also drops support for PHP 5.2 but I don't think this is really a problem, PHP 5.3 has been here for more than 5 years and its support is going to be deprecated anyway, so supporting PHP 5.2 would be pointless.

Thanks !