Add include_media_info to `metadata`

On https://www.dropbox.com/developers/core/docs#metadata, the dropbox API lists `include_media_info` as a way to get the time-taken from photos (as well as other photo/media information).

This adds that as an option, defaulting to true. 

Thanks! (this is my first pull-request, so if you'd like anything else, please let me know)
I agree with that! Including media info can actually slow down metadata requests a bit (especially on large folders) so we don't want to doing that work unless the developer actually wants it.  Thanks for updating this though!