Laravel 5 version

Updated the package to support Laravel 5 (>=5.0.30).

Other improvements:

• Some configuration options via .env file (token, base URL and enabled)
• PSR-4 instead of PSR-0
• Updated to Guzzle 6, with support for PSR-7
• Conversion from PSR-7 Messages to Symfony Responses is now done using Symfony's psr-http-message-bridge.