Implemented HttpAdapter with guzzle

Signed-off-by: Graham Campbell <graham@mineuk.com>
Added http adapter docs to the readme
Merge branch 'guzzle' of https://github.com/GrahamCampbell/flysystem into feature/http-guzzle
Some restructuring and removed unused use statement
Merge branch 'master' of github.com:thephpleague/flysystem
Updated rackspace docblocks