Updated Tests, Use Guzzle

I updated some old unit tests which failed due to the port being changed over to :80 from :8080 previously.

I also, mainly, ported the ApiClient over to use Guzzle, which is faster (curl-based with a file_get_contents fallback). Additionally, with that, I re-factored some existing code which was a bit shaky (namely the manual creation of a JSON string).

In order to test that Guzzle was working correctly, I removed the Good/BadMockApiClient from the test and just used the live ApiClient. The mocks did not actually test the request itself. If you want to reinstate something like that I'd suggest using Mockery and inserting a Guzzle client mock into the constructor of the ApiClient class - that'll give you the best results.

(by the way, I just want to thank you for having written one of the only decently coded PHP classes I've seen related to Minecraft stuff)