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 think the `include_media_info` option should default to `false`. (The Core SDK docs say "if true, each file will include...", so true shouldn't be on by default.)