Bugfixes

Thanks for the library, it saved me a bunch of effort! Here are two bugfixes for things that I ran into while using it.

1. Rn doesn't get set for thumb2 instructions. This looks like an oversight.
2. CBNZ and CBZ are decoded incorrectly.
There's a mistake in the bitmasking here. 
From the reference manual:

![screen shot 2014-10-27 at 11 15 08 pm](https://cloud.githubusercontent.com/assets/593995/4804412/8d073040-5e6b-11e4-8467-00bd4012a9ea.png)