New N-WriteStream, N-ReadStream and Device Example

NanomsgReadStream now uses the UnmanagedMemoryStream class.

NanomsgWriteStream now writes into a MemoryStream; with Flush it
prepares a nn_iovec struct and copies the current MemoryStream buffer
with Marshal.Copy into an nn_allocmsg allocated buffer.
The entire message is built in NanomsgSocketBase.SendStreamImpl.
The inproc protocol works as expected.

I've added a Device example, which has multiple REP workers answering to
multiple REQ messages in parallel.
Also try it with inproc protocol, by commenting-out the const string
host for tcp.

I am submitting this patch under the MIT license.