[WIP] gs module: add backward-compatible support for Python 3.3+

All integration tests already failed with Python 2 for me, and my port is getting same failures. Still figuring out why.

Sample:

```
======================================================================
ERROR: Test bucket and key ACLs.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/felix/projects/boto/tests/integration/gs/test_basic.py", line 223, in test_acl
    bucket = self._MakeBucket()
  File "/home/felix/projects/boto/tests/integration/gs/testcase.py", line 84, in _MakeBucket
    b = self._conn.create_bucket(self._MakeBucketName())
  File "/home/felix/projects/boto/boto/gs/connection.py", line 104, in create_bucket
    response.status, response.reason, body)
GSResponseError: GSResponseError: 403 Forbidden
<?xml version='1.0' encoding='UTF-8'?><Error><Code>SignatureDoesNotMatch</Code><Message>The request signature we calculated does not match the signature you provided. Check your Google secret key and signing method.</Message><StringToSign>PUT
```
You could do `from boto.compat import six, http_client` above to save some typing here and below.