Clearer password guidance

We have received reports from users about not being able to change their passwords as a result of the tighter password restrictions introduced by #122. The first attempt to fix this issue is to provide better guidance about valid and invalid passwords.

This is what the guidance looks like on the "passphrase expired" page:

![image](https://f.cloud.github.com/assets/23801/1940329/709cceb8-7f75-11e3-86d5-e4f8cdc17203.png)