Add a Guzzle client

[mattjmattj]

Add guzzlehttp/guzzle as composer suggestion and provide a
client based on it.

Nothing fancy here, but hopefully the first step toward asynchronous requests in essence/essence

[mattjmattj]

Whoops. Travis tests fail because Guzzle is only a suggestion, not a dependency. Maybe should I had it as a require-dev dependency

[felixgirault]

Thank you!
Though I think the Essence\Http\Client interface should live in the main repo...
This way we could create a new repo with your code, and the user could choose what client to use.