Add Havana support

(I've described this as a hack because it drops the metadata_keys support. We don't use it, so I didn't spend much/any time looking for an alternative. Also I set a null memcache in the copied request to avoid "memcache not in  env!" message in the log.

Wrt: how to copy orig_req --- I started subclassing swob.Request and moving webob's copy code into the subclass. However, swob.Request.blank() creates swob.Request ... not my subclassed class. Hence the solution proposed.  I didn't spend time on the unit tests in anticipation that a better/alternate approach might supersede them)

Use swob instead of webob.

Not complete:

- The container metadata_keys functionality is broken because
  swift/container/backend.get_info() does not support.
- The unit tests not updated (waiting to see if this general approach
  to using swob is ok).