Dropbox integration [#4]

Connect to Dropbox instead of using the local file system.

Closes issue #4

* Updated README with how to setup Dropbox
* Created a support file to stub out Dropbox API calls
* Added Rake task to be able to retrieve a user's Dropbox access token
* Dropbox reader to read files from a Dropbox store
* Dropbox iterator to iterate over files from a Dropbox store
* Dropbox output store to save files in a Dropbox store