Adding support for Guzzle 4

Converted to Guzzle 4. All existing tests pass.

- Added support for the new Guzzel 4
- Added event config options for the Http Client (Before, Complete and Error)
- Added option to pass the `verify` option to fix issues with PHP 5.5.14+ on Mac OS X (homebrew)
please follow Symfony coding standards and don't namespace classes in the global namespace