Start with a kind of "standard library" for SMC scripting.

While the SMC scripting "core" is what is hardcoded in the scripting
C++ source files, the stdlib contains additional stuff written in
Ruby. That may be convenience classes, like the ExitDoor class this
commit adds, or mixin modules, or whatever. Additionally, script
writers can publish their own "SMC scripting packs" and have users
place them under the scripting/ directory right next to the std/
directory for the stdlib. This hopefully allows for a vivid
scripters community.