Private messages from @hardstylle

PM parsing and viewing extracted from hardstylle/PrivateMessages (#165), will continue from here.

- [x] Use a posts view or something similar to show a message.
- [x] Compose screen. (Didn't bring in wysiwyg composer.)
- [ ] Update missing thread tags in messages list.
- [x] `awful://messages`
- [x] Check `awful://settings`, might've hard-coded to fourth tab.
- [x] Document new methods in `AwfulHTTPClient`
- [x] Badges on tabs. (Easy way: go back to UITabViewController or find someone else's custom one.)
- [x] Check for new private messages (poll?).
- [ ] How does it look for people without platinum?
- [ ] Handle errors when sending (e.g. intended recipient hasn't platinum)
- [ ] Folders?
- [ ] Show all (default only shows 50 messages).
- [x] Parse everything when viewing private message (username, custom title, regdate, sent date).
- [ ] Dark theme for PM list and view (including posts view background).
- [ ] Two-line title for PM view.
- [x] "Post" actions in PM view? (e.g. reply, show profile)