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
Add "Must be at least 8 characters" to password fields.

fixes #264
Prevent users from selecting text in the password show/hide field.
Merge pull request #280 from mozilla/issue-253-show-passwords

password updates - Show/hide passwords & 8 character min help text.