Edit the secret key during registration

Allow editing the secret key during registration.

@wynlim UX feedback?

Fixes #99.

![screen shot 2014-10-27 at 1 51 40 pm](https://cloud.githubusercontent.com/assets/3108007/4799310/70c108f2-5e1b-11e4-9a27-1fc0fd1181a0.png)
![screen shot 2014-10-27 at 1 51 57 pm](https://cloud.githubusercontent.com/assets/3108007/4799309/70bccf80-5e1b-11e4-8a8c-b4fe52cee02c.png)