Interface to zmq_proxy()
Correctly define ZMQ_DELETED_FUNCTION when using older clang compilers.

If you are using a version of clang that doesn't support cxx_deleted_functions
we need to define ZMQ_DELETED_FUNCTION to be empty.
Merge pull request #13 from jdswinbank/master

Adding a wrapper for zmq_proxy()
Merge pull request #14 from robertmaynard/master

Define ZMQ_DELETED_FUNCTION on older clang versions.
Merge branch 'master' of git://github.com/zeromq/cppzmq