Add split view, big reorganization, and some refactoring

* Includes a split view implementation on iPad for filtering tasks, still limited to landscape mode like my last PR. The app should look nearly the same as before, aside from the new split view.
* Task cells are laid out via Auto Layout in code.
* Most of the files have been reorganized and most xibs have been consolidated into the storyboards.
* Thanks to the refactoring and move to Storyboards & Auto Layout, a significant amount of code has been removed.
* ReactiveCocoa has been added as a dependency via git submodule. It is used in a few places.
* Updated the Dropbox SDK to 1.3.4

Aspects you may want to change:
* The fonts for the priority and age labels may have changed
* The splash screen displayed when the user has to log in to Dropbox could be laid out a bit better
* The help popover when editing a task on iPad could look better

None of those should be difficult, but I think they come down to taste so I'm leaving them for you to change as you see fit.