CMake: used ZMQ shared lib for opentxs shared lib

In the switch to use the system's libzmq we preserved the
statically linking of it. But since all other library dependencies
are linked as shared libraries libzmq should be as well.
Then, third party library linking is consistent when building our
own shared library.
Merge pull request #6 from toxeus/sharedZMQ

CMake: used ZMQ shared lib for opentxs shared lib