Moving zerorpc to an entry_point

When using 'scripts', the scripts are installed as-is, which is a problem with:
* easy_install-in on Windows -- no executable wrapper is created
* buildout environments -- no wrapper is created

When using console_scripts entry points, all of these are created just fine.
The switch is simple and easy and does not break anything, but it does moving the 'zerorpc' script inside the package
Add back bin/zerorpc, now a tiny wrapper.

This small wrapper is here when you want to run zerorpc without
installing it, for some testing or any specific development
environment.