Use Guzzle implementation #20

Currently we have implemented the PSR classes:

• Message
• Request
• Response
• Uri

with our own implementation. This was done to have not a direct dependency on Guzzle. Since we already use the PSR interfaces we could also simply extend these classes from the Guzzle Implementation. Through this we dont need to maintain our own implementation anymore and we still have our own class which we can extend with methods. This means also we can remove our tests since Guzzle has a great test-suite.