Fix response parsing for get-bucket-policy

This is the case where we have a single output member,
and that member is marked as a payload, as we see in
GetBucketPolicy.
Fix py3 bug in xml response parser
Fix py26 bug in xml parser

So in py26 invalid xml raises a SyntaxError.  Updated
the compat module to hide this difference.

Verified all tests pass in py26, py27, and py33.
Merge branch 'get-bucket-policy-fix' into develop

* get-bucket-policy-fix:
  Fix py26 bug in xml parser
  Fix py3 bug in xml response parser
  Fix response parsing for get-bucket-policy