Add rawBody for proxy and XML requests

I stole most of this from @kristjan's branch, here's his commit message:

> Express/Connect has removed both the old `req.rawBody` parameter (for memory
> reasons) and the ability to add custom parsers for specific MIME types (for
> unknown reasons). This reintroduces the `rawBody` field for `POST`s to us only
> of type `application/xml` (for minimal impact reasons) so that we can pass it
> along to Shutterfly. Since we don't actually care what's in it, I haven't
> bothered to parse it.

I also added rawBody for all /proxy/* requests.
Fixes for dropbox proxy