Migration to Toolbar, and 5.0 support <3

- Upgrade target SDK to 21
- Upgrade build-tools to 21.1.1
- Upgrade support library to 21.0
- Implement Toolbar to take the place of the "old" ActionBar
- Delete drawables and styles related to the old ActionBar
- Bring back the old quick-return handler, since the Toolbar is a regular
  ViewGroup now.

Notes:
- I gave the Toolbar a manual drop-shadow, since the AppCompat library
  does not yet support the "elevation" api. Once it becomes supported, we
  can use setElevation().
- The Settings window (PreferenceActivity) doesn't have an ActionBar,
  since that's not supported by the AppCompat library either. We may need
  to manually give it a Toolbar, just like in the Main activity.

Change-Id: I9a00c49dd2544d7d2b1990028db2fb8ee1ce97d4