Add receive.multipart and send.multipart functions

Hi, I'm using rzmq to write an R kernel for IPython. I wrote these functions as part of the kernel, but I think they belong in rzmq. It may be more efficient to implement them in C++, but I'm even less familiar with C++ than R.

I find the API somewhat confusing - there are several `receive.*` functions without send counterparts, I'm not sure if `receive.string` and `send.raw.string` are counterparts or not, and the serialize/unserialize parameters to `receive.socket` and `send.socket` seem to make the other send/receive methods unnecessary. For my purposes, I serialise the data as JSON before using rzmq, so I'm only interested in sending and receiving bytes/strings, but I'm happy to make the API more flexible if that makes sense.