Add readdir options: recursive, details

[benbuckman]

This adds an options parameter to client.readdir, with 2 options:

• recursive: Added because a recursive traversal of a user's entire Dropbox can be terrible for performance. In our case we want the user to traverse manually only as deeply as needed.
• details: if true, returns the whole item object for each item, not just the path.

All of this is added in a completely backwards-compatible way, maintaining the original behavior for users who don't explicitly specify these parameters.

[sintaxi]

Sorry for the late reply. I like this commit. I will try to put together a release shortly.

[benbuckman]

Thanks!