Improves homepage appearence and mark up.

* Uses more semantic HTML (e.g. a heading for the welcome message)
* Adds Bootstrap 3 row markup to take advantage of grid system
* Removes un-needed padding for #welcome-message
* Added a center-block class to log / sign up buttons, so
  they render nicely on seperate in smaller viewports. A media
  query then gives these elements display inline on larger devices
  for cleaner UI.
Removing custom get_user_or_404() function in favour of built in get_object_or_404()
Removes BodyWeightForm processing in nutrition.py

* We actually update the BodyWeight table via our REST API so this
  code is not needed
* Found when writing tests for django views
Introduces a simple 404 template.

* Needed in production when DEBUG is set to False
Found a rogue URL string in nutrition() - we should be using reverse() and the URL name attribute instead