Scriptzeug: Duktape as Javascript scripting backend

This adds [Duktape](http://duktape.org/index.html) as Javascript scripting backend to Scriptzeug.
Duktape is a more compact/lightweight alternative to V8. The two source files duktape.c and duktape.h are already included in the build, which removes the dependency of a big library like V8. ref #79