Support Guzzle4

I initially though we (you or me) would have to create a new repo for Guzzle 4 support because of the dependancy to guzzle (which is required for tests).
But because Guzzle has actually renamed their package from `guzzle/guzzle` to `guzzlehttp/guzzle` it's possible to have both versions in `vendors` and we can support both versions of Guzzle with 1 repo/branch.

Guzzle 4 has a different API so I've added new classes in the `HttpSignatures\Guzzle4`.

Let me know what your prefered way of organizing this would be and I'll follow ;-)