Smilie keyboard + extension

* [x] Add [custom keyboard extension](https://developer.apple.com/library/ios/documentation/General/Conceptual/ExtensibilityPG/Keyboard.html) and [smilie](http://forums.somethingawful.com/misc.php?action=showsmilies) framework.
* [x] Figure out smilie scraping and persistence.
    * [x] Ship smilies with Awful.
* [x] Implement keyboard extension.
    * [x] Barely-working basic functionality: show 100 smilies and copy the image data on tap (requires "allow full access" in Settings).
    * [x] Custom collection view layout, for better alignment and pagination.
    * [x] Show all smilies, broken into sections.
    * [x] Delete key.
    * [ ] Notify when "allow full access" is off.
* [ ] Integrate smilie keyboard into Awful itself.
    * Make it available even when extension is turned off.
    * Insert `:wotwot:` instead of ![](http://i.somethingawful.com/forumsystem/emoticons/emot-wotwot.gif) (regardless of how it's showing in Awful).
* [ ] Periodically download new smilies.
    * [ ] Delete downloaded smilies when app update includes them. But keep the metadata around.
* [ ] "Recently used" section.
    * [ ] Sync using iCloud?
* [ ] Vend PONSOs as public API instead of managed objects?
* [ ] Intercept posts/messages view requests for smilie images and serve them from our database?
* [ ] Avoid duplicate class definitions for HTMLReader in the Awful app target.
* [ ] Remember which page of each smilie section we were on, and jump back to that page when loading that section.
    * Good use of that app group user defaults domain.

## Prior art

@hardstylle got started on this ages ago in #175. The code there is hopelessly out of date by now, but take a look for inspiration and scraping.