Add home page.

The home page was previously just the default Railsbricks starting page.
This commit adds a customized home page, allowing users to subscribe to
notifications.

Todo:
  * update `Subscribe' and `Update me' buttons to fit better with theme.
  * update `About' content
  * add email update functionality (is in view now, but doesn't work)
  * remove `See the tour' and `Sign up free' buttons, and change to
    something else.
  * remove spacing below `Getting notified is easy.'
  * update brand name