Private messages from @hardstylle

PM parsing and viewing extracted from hardstylle/PrivateMessages (#165), will continue from here.

- [ ] Use a posts view or something similar to show a message.
- [ ] Compose screen. (Didn't bring in wysiwyg composer.)
- [ ] Update missing thread tags in messages list.
- [ ] `awful://messages`
- [ ] Check `awful://settings`, might've hard-coded to fourth tab.
- [ ] Document new methods in `AwfulHTTPClient`
- [ ] Badges on tabs. (Easy way: go back to UITabViewController or find someone else's custom one.)
- [ ] Check for new private messages (poll?).
- [ ] Folders?