[WIP] Make Apple Pay sheet look almost identical to Apple's implementation

I'm in the UK where Apple Pay isn't yet supported, and need to demo something closely resembling the legit Apple Pay sheet. I've replicated the UI almost perfectly based on this screenshot: https://www.crittercism.com/wp-content/uploads/2014/10/Screen-Shot-2014-10-21-at-1.31.40-PM.png

It makes use of `PKGlyphView` to render the fingerprint icon (complete with tick animation). Note: this is a *private* API.

<img src="https://cloud.githubusercontent.com/assets/363540/5210224/82fa87c0-75c0-11e4-9990-ed647db5786d.png" width="375">

Still to do:
 * Sheet needs to automatically scale to correct size
 * Greyed out background
 * Sheet should dismiss when transaction is complete