Show/hide passwords on sign_up, sign_in, and complete_reset_password

* Add a styled checkbox to each password field that toggles the show and hide.
* To show a password, change the input field type to "text"
* Add unit tests for the affected views

fixes #253

It looks like:

![screen shot 2014-01-17 at 15 45 19](https://f.cloud.github.com/assets/848085/1941764/985cc54c-7f8f-11e3-97e2-32f2653304d5.png)

![screen shot 2014-01-17 at 15 52 48](https://f.cloud.github.com/assets/848085/1941767/9b17e2a8-7f8f-11e3-97cd-a6b6d3febb8d.png)