User cache now works correctly.

Partially resolves issue #76.

* The function `User.getMyUser()` stores a cached version of the user so that
  it doesn't need to read the user entity from the datastore every time the
  function is called. However, this function wasn't working because it was
  comparing the cached user ID against the App Engine user ID using the `==`
  operator. It now uses the `String.equals(String)` function, and it now works.
* Deleted old files which aren't used anymore. Probably should have been split
  out into a different commit.