Recursive drop_get

I might be missing something but I don't see support for downloading entire directories? It looks from a [random bit of googling](http://stackoverflow.com/questions/20227324/programmatically-download-content-from-shared-dropbox-folder-links/20243081#20243081) suggests that this could be possible but that possibly falls outside of the API?

When calling `drop_get` on a directory I get json saying:

``` json
{"error": "Path is a directory"}
```

though drop2 does not convert this to an error (I suggest a `httr::stop_for_status(x)` after the `GET` as dropbox is sending a 500 status with the response).

Version: 0.7.0 (CRAN) which looks close to the current GH version.

Unrelated; the authentication dance is _slick_. Really nicely done.