Update README to use session to store request token
Update README to use session to store request token

The README was mysteriously blank as to how to use the request_token
since the user is redirected to a different site.  This fills in the
proper way from the OAuth classes.
datapimp/dropbox-api,futuresimple/dropbox-api,unbox/dropbox-api,petems/dropbox-api-petems,mikicaivosevic/dropbox-api