Some small cleanups.
Moves topic and rpc data into structs.
Don't leak callback messages.
Removes incorrect check.
Adds node::listen()
Adds gtest.
Adds empty first test.
Check thread joinable in ~ZeroConf
Fix Node destructors.
Adds send, listen and string pub/recv methods.
Adds protobuf, zmq message send calls.
Adds NodePubSubTest.
Context only needs one thread.
Remove commented out lines.
Lots of refactoring to Node.h and a couple to .cpp
Remove redundant return.
Use lambda in provide_rpc!
zmq::context no longer a singleton. Other cleanups.
Set socket linger to zero.
Merge commit '9d3e82954b0ae6414904bc8a2529c1603d802ae8' as 'zmqpp'