rebase of #21 fix for rails 4

This is a rebase of #21 that let's `has_mobile_fu` be used in ApplicationController, and properly falls back to html views when no mobile override is present. It should also work with Rails 3 but has not been tested. It likely will *not* work with Rails 2, as the method signature for the method it overrides has changed (the issue that stalled #21 in the first place).