Add an option to mput that will upload a file using the Manta multipart upload API

[jordanhendricks]

Once the multipart upload code is integrated into master on manta-muskie, it would be nice to have a command line tool that just takes a file as input and does the process of a multipart upload for you, including chunking the file up into parts as needed.

I could see this as its own m* command in node-manta, but I think it might also be nice to just provide a flag to mput (perhaps --multipart), which, combined with -f, will chunk the file and do the multipart upload process for you.