Password reset

Add ability for users to reset their passwords via email. Also allows users to edit their email and passwords within the app. Resolves #13 .
I'd wrap all this in a separate class that isn't backed by a database -- something like:

```ruby
reset = PasswordReset.new(:email => 'user@host.com')
reset.deliver # create reset token and send email

reset = PasswordReset.find('token')
reset.expired?

reset.password = 'password'
reset.password_confirmation = 'password'
reset.update_user # validate passwords match, update user, delete reset token
```

Take a look at the documentation for [ActiveModel::Model](http://api.rubyonrails.org/classes/ActiveModel/Model.html) for how to create an object that has validations and can interact with `form_for`.