[Http] Make http transport configurable

Hey!

This PR fixes #71 by relying on `egeloen/http-adapter` which allows to use the http library you want according to your project dependency. Then, if you don't have curl on your test server, you can rely on `buzz`, `guzzle4`, `zend`, `socket` and others.

Give me your feedbacks!
I will use the `~3.7` constraint and guzzle 5 is not yet supported as you can see in egeloen/ivory-http-adapter#35