add a call stack to the Interpreter (refs #98)
- looking up classes and subroutines now also returns the class from which they were found. Also will search the root package when it makes sense.
- MoePackage and MoeClass now have getFullyQualifiedName methods
- change all usages of lookup{Subroutine,Class} to understand the changes above
- implement call stack tracing
add the caller builtin and a test for it