Replaced Guzzle with an abstract way of making HTTP requests

Hi there,

I have replaced the Guzzle library with a more abstract HTTP adapter for the following reasons:

* Do not have a dependency on CURL if you don't have to.
* Allow the user to choose the HTTP library (not to have both guzzle 3 and 4 alongside for example)
* Be compatible with the upcoming PSR-7 standards (https://github.com/php-fig/fig-standards/blob/master/proposed/http-message.md)

Hopefully you guys like the idea :+1: