Style the typed and the correct texts.

When the answer is typed, use the INSERT/DELETE/EQUAL results that the diff engine supplied all the time to style the input *and* the correct texts, similar to the way Anki desktop does now.
Clean up (trim trailing whitespace &c.) the typed and correct texts a bit.

Seems to run with Android 2.1 devices. :persevere: At least on the emulator. Just the unicode normalization that [Anki desktop does](https://github.com/dae/anki/blob/master/aqt/reviewer.py#L446-L447) is skipped. So this hopefully avoids  something like  [this](https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/anki-android/luiSvDOrkZQ/_aW54vvAc2oJ) problem.

Demo: Anki desktop:
![barbados_desktop](https://cloud.githubusercontent.com/assets/1690429/2984296/a8b9b096-dc30-11e3-9d34-d7438aca2d49.png)

AnkiDroid before:
![barbados_before](https://cloud.githubusercontent.com/assets/1690429/2984382/bc829c9a-dc31-11e3-9681-897fe6d87f28.png)

AnkiDroid after:
![barbados_android](https://cloud.githubusercontent.com/assets/1690429/2984389/c5835172-dc31-11e3-9712-d5125513d945.png)

(Emacs removed some trailings spaces.)