add lua support to write rules

[Gaëtan Lehmann]

a lot of things already done in https://github.com/purpleKarrot/Vera--

[Alejandro Claro Mosqueda]

It would be nice multi-interpreter support. My team would love write the rules in Python. Lua is a terrific idea too.

[Gaëtan Lehmann]

first lua work - see #26

still at least need to: * reorganize Interpreter.cpp * display the stack trace in case of error * add an option to statically link with lua and luabind

→ <<cset 89d32437f9b4>>

[Gaëtan Lehmann]

this is fully supported now!