Add support for sending Uint8Array objects

[ameily]

I added support for sending Uint8Array objects over a zmq socket. The problem was that zmq was wrapping the Uint8Array object in a Buffer before sending the object out.

[ronkorving]

Hi @ameily, thanks for the contribution.
Does the handling of that Uint8Array in C++ land work?

[ameily]

I'm not entirely sure how Node implementes the Uint8Arrayclass in native code. However, I've verified that this PR works and the zmq library properly handles Uint8Array. I'm using flatbuffers, which is a binary protocol that uses Uint8Arrayobjects and everything seems to work fine on both the sending and receiving ends.

[crostagnol]

Thanks @ameily for this contribution! It was very helpful for my project.
I'm also using flatbuffers over ZMQ.
It would be great if this commit is merged into the master branch for next release ;-)