send.multipart and receive.multipart work with lists of raw vectors

zmq data is not guaranteed to be convertible to strings - in particular,
it can contain embedded null bytes, which rawToChar chokes on.

I'll fix docs and tests in this PR tomorrow - this is a late-night
change to fix a problem downstream.
Update docs
Merge pull request #8 from takluyver/multipart-raw-vectors

send.multipart and receive.multipart work with lists of raw vectors