Remove import of unused header.
Add BTRSecureTextField to text field demo app.

- Xcode first tried to upgrade the XIB to 5.1 and up only (silently of
course). I’ve set the IB compatibility setting to Xcode 4.6.
- I’d argue that it should be raised to 5.0 or 5.1 as the XIB becomes a
whole lot better then.
Refactor out the BTRTextField API into protocol.

- This is so that it’ll be easier to keep BTRTextField and
BTRSecureTextField in sync.
Sync BTRSecureTextField implementation with BTRTextField.

- They were quite a lot out of sync.
- Both now use a shared protocol to define the shared public API. This
should hopefully make it somewhat easier to spot if this happens in the
future.