Bucket listing support

* Buckets listings can now be retrieved
* Bucket listing tests
* Updated to em-http-request 1.0.x series
* Replaced EM::MockHttpRequest, which is no longer available, with webmock
* Reduced test timeouts from 1s to 0.1s for faster test runs
* Added parser option to Request for parsing S3 responses
* Use gemspec for test dependencies