Fixes #174.

account.html:

- Add a new "Change Your Email Address" form

accounts/views.py:

- Add email to the user dict returned at /app?__formid__=profile (or
  session.profile() in Angular)

- When resetting the form after a successful submission, put the new (or
  unchanged) email address into the user dict returned in the response.
  AccountController needs this (see below).

account-controller.coffee:

- Add $scope.email (the user's current email address), to use for placeholder
  text in the email form field.

- After a successful form submission set $scope.email to the one from the
  response. This sets the placeholder text in the email field to the new value,
  instead of continuing to show the old email address as placeholder.

- Add changeEmail() method to receive email form submissions.