[Http] Make http transport configurable

This PR is a fixed version of #73 which now relies on the last `psr/http-message` spec. I still need to investigate a strange behavior (work locally but seems not working with travis...). Furthermore, I'm not sure about the composer constraint for `egeloen/http-adapter`. As we are in pre `1.0.0` release, I break BC in minor versions making potentially the `0.5` not compatible with the `0.6` versions so, I have decided to fix the constraint to the `0.5.*` releases and then, if it is compatible we can bump the constraint on the next release. What do you think?