Rewrite of PKView for iOS 7

Big update for `PKView` with many cleanups and refactoring across the board, and UI changes to bring the look and feel in line with iOS 7.

![paymentkitnew 2x](https://f.cloud.github.com/assets/363540/1820885/50ae7412-7110-11e3-8144-7b3522eaab01.png)

 I have a few more changes to come tomorrow affecting expiry/CVC behaviour.

Note: this breaks compatibility with the old style `PKView`.