Prevent use of uninitialized variable.

Problem: `wait()` wasn't safe, if `receive()` failed,
if would use an unitialized message.
Solution: Make sure received didnt fail.

Problem: If `receive()` was interrupt (EINTR) actor
constructor would use a non-initialized signal, causing the
assertion to fire.
Solution: Replace call to `receive()` by `wait()` in actor constructor.
Merge pull request #92 from zeromq/uninit-var

Prevent use of uninitialized variable.
CMake can depend on libzmq cmake.

Add a CMake flag (ZMQPP_LIBZMQ_CMAKE). When set to true (which is
not the default value) this flag instructs CMake to link against
libzmq built through CMake: cmake will now properly know that
zmqpp depends and libzmq and will not fail if it can't find it
in the path.
Edit link library.
Merge pull request #94 from xaqq/update-cmake

Update cmake