Added content_hash to FileMetadata and options to list_folder request

[kitebuggy]

Hi,

A couple of additions for your consideration, please:

• Content_hash information allows us to check locally if the file has been changed since the previous upload (using the DropboxContentHasher gem, for example). This PR add this field to FileMetadata.

• The list_folder API call has some useful options, e.g. recursing, listing deleted files, etc., so these options have been added into this method.

• Optionally, I've cheekily included emacs save files to the .gitignore file. :-)

Thanks,

Jason

[jamespearson]

Thanks for this, hopefully it will get merged in soon