Major touchup

* Added namespace
* Added docblocks
* Refactored code for more readability
* Improved composer configuration
Add explizit factory class
Improve request creation
Remove file handling

StreamInterface::addFile method does not exist, so the file handling
never worked anyway.
Removed explizit host header removal

RequestInterface::setUrl overwrites the host header for us
Update documentation and remove null option from constructor
Add license and add proper version constraints
Update documentation and remove null option from constructor
Move request/response convertion into own classes
Move request/response filter into own classes
Merge pull request #1 from rebuy-de/converters

Converters and filters
Remove Request object generation

The user should be in charge to create a request object, dont create it
automagically.
Combine request and response converters into adapters
Merge pull request #2 from rebuy-de/transports

Combine request and response converters into adapters
Rename namespace from Phpproxy to Proxy
Merge pull request #3 from rebuy-de/namespace

Rename namespace from Phpproxy to Proxy
Drop htaccess file
Gotta rename 'em all
Various Bugfixes and small improvements
Guzzle goes optional