UpdateWithMedia now accepts a slice of byte slices

I have created a branch where UpdateWithMedia accepts a slice of byte slices rather than a slice of filenames. In this manner, media can be uploaded to Twitter from any source (file, URL, local data in memory, etc) rather than only a file.

This is a backwards-compatibility breaking feature and I propose that it be considered for the next full release of your library. In the meantime, I have set it live in my master branch at github.com/carbocation/twitter . Cheers!