adding upload media endpoint

I started having some problems with update_with_media method and it looks like it's deprecated:
https://dev.twitter.com/rest/reference/post/statuses/update_with_media

The new method to update with media is described in here:
https://dev.twitter.com/rest/public/uploading-media-multiple-photos

This pull request adds ```upload_media``` method to twython so images can first be uploaded to twitter before they are attached to a update.   
I also added to docstring an explanation on how to proceed after uploading the media.