Add Dropbox Persistence Method

[doolittle]

An option to write to Dropbox would be a useful alternative to GitHub.

Users could load notebooks from a shared folder and write them back to a shared folder (for group activities) or save them to a private folder.

[blakeembrey]

Is this an alternative persistence layer, or a concurrent option?

[doolittle]

Alternative.

[blakeembrey]

This would actually work really well since (unlike Github) Dropbox supports implicit (client-side) OAuth2. Which means we can have a persistent version deployed to Github pages at all times.