Update README.md
Fixed unit test to match Guzzle configuration parser change
Fixed EmbeddedCommand to extend AbstractCommand, fixed tests accordingly
Fixed some interface usages to be more flexible
Updated Guzzle to 3.7.4 (sorry, 3.8.0 is not stable enough about additional properties in JsonVisitor)
Merge branch 'update-guzzle' of github.com:David-Guillot/desk-php into update-guzzle