dropboxjs: Allow using as an external module

Actually, I sent this PR a bit too fast. After using the module more, I noticed that:

- It should be renamed from dropboxjs to dropbox
- The version published to npm by Dropbox is severely outdated, eg. the Datastore API is totally missing from it.

So I guess the way forward would be for me to rename this module, rebase this PR on top of the "rename" patch, and push the bindings I have added locally for the Datastore API along with the tests.