[1.2] Migrate from Buzz to Guzzle

From #30 
```
The improvement is pretty good : when running multiple request in the same PHP script, 
Guzzle use CurlMulti as base request handler, using HTTP 1.1 at its best 
(all requests are done inside the same connection) whereas Buzz does 
not support it (it opens a new connection on every request).

Here are the results of my mini benchmark for 50 requests :

Buzz ~ 1min
Guzzle ~ 15seconds
```