Support for gzip compression

API calls with verbose results can take a very long time when made over network links with high latency and/or low bandwidth. The API endpoints seem to support gzip compression, but it is unused by the SDK. This pull request adds support for gzip compression. 

Example:
* A `describeInstances` request to an account with about 1,000 instances returns about 6MB of uncompressed data. Over a slow link in New Delhi, this single API call takes about 1 minute 30 seconds. 
* With gzip compression enabled, it takes 11 seconds and transfers only 246 kB).