Change guzzle version constraint to 3.*

Guzzle will be moving the backwards-incompatible Guzzle 4 to
dev-master soon, so we'll constrain this to 3.* to prevent issues in the
future.
Add failing test for Pockpack::send
Set request details in query string.

Guzzle doesn't support setBody on GET requests. Instead we'll send the
details in the query string, as specified in the [Pocket API
docs](http://getpocket.com/developer/docs/v3/modify).
Merge pull request #28 from jwpage/fix_pockpack_send

Fix pockpack send