Modernize views, view controllers, and other classes.

[brendonjustin]

• Use properties where possible, and avoid declaring ivars
• Replace most nibs by using Storyboards
• Replace FlexiCell and supporting classes with one class that uses Auto Layout
• Make cells for displaying tasks using Auto Layout and ReactiveCocoa
• Reorganize project structure to group classes by role
• Update README.md with instructions for init'ing the ReactiveCocoa submodule and adding a DropboxApiKey.h file
• Add ReactiveCocoa v1.9.6, as a submodule, and use it to simplify use of DropboxFileDownloader & Uploader

Also add a view for filtering tasks.

[brendonjustin]

Did an amend/forced update to add a missing timeout in most downloader/uploader tests.

[ginatrapani]

Awesome, thanks Brendon!