This change replaces WebOb with a mostly compatible local library,
swift.common.swob. Subtle changes to WebOb's API over the years have been a
huge headache. Swift doesn't even run on the current version.
There are a few incompatibilities to simplify the implementation/interface:
* It only implements the header properties we use. More can be easily added.
* Casts header values to str on assignment.
* Response classes ("HTTPNotFound") are no longer subclasses, but partials
on Response, so things like isinstance no longer work on them.
* Unlike newer webob versions, will never return unicode objects.
Change-Id: I76617a0903ee2286b25a821b3c935c86ff95233f
