Added changeWorkingDirectory() and getWorkingDirectory() to FileSystem
Added a FileSystem\Path class as an OO alternative to FileSystem\FileSystem
Refactored Http\Messsage, Request and Response

- Moved some functionality to Message, harmonized message bodies
- Made Message, Request and Response compatible with upcoming PSR standard
- Added setters for most properties
- Made Request::getCurrent() faster
- Added configurable host and port sources to Request::getCurrent()
- Added unit tests for Request