Make it possible to set bug privacy using the API

[Kees Cook]

Expose IBug.setPrivate to that it's possible to set bug privacy using the API.

--------------------------------------------------
was:

*setup launchpadlib object*
bug = launchpad.bugs[num]
bug.private = False
bug.lp_save()

this fails:
Traceback (most recent call last):
  File "./not-a-bug.py", line 33, in <module>
    bug.lp_save()
  File "/usr/lib/python2.5/site-packages/launchpadlib/resource.py", line 467, in lp_save
    self._root._browser.patch(URI(self.self_link), representation)
  File "/usr/lib/python2.5/site-packages/launchpadlib/_browser.py", line 263, in patch
    extra_headers=headers)
  File "/usr/lib/python2.5/site-packages/launchpadlib/_browser.py", line 211, in _request
    raise HTTPError(response, content)
launchpadlib.errors.HTTPError: HTTP Error 500: Internal Server Error

content is: ('private', <Bug at 0x974f1d0>)

OOPS: OOPS-1081EA142

[Eleanor Berger (intellectronica)]

Setting bug privacy using the API is not possible yet. When it will be, though, it probably won't be by setting the `private` attribute but rather by calling setPrivate on the bug.

[Kees Cook (kees)]

In the meantime, the documentation should be adjusted -- it currently shows "private" as "writable".