Add runtime czmq version check function

This change adds a function to provide CZMQ version details at runtime. This is useful to libczmq users that need to perform version checking at runtime, not compile time. One example is pyczmq which is not linked against libczmq but rather loads libczmq at runtime via the ffi module. 

Adding the version function to the zsys module seems to be the best spot for now.

The README.md was not up to date with the current repo state so when I regenerated it to include my changes it has also added some recent changes that were missing.