Added help function support for modules

[Vardan2009]

Closes #164

This adds support for printing module contents by the help(obj) function

But, I'm not sure about these lines in the code:

if (type(f).__name__ == "BuiltInClass") or (type(f).__name__ == "BuiltInFunction"): continue
if k == "null" or k == "false" or k == "true": continue

[Almas-Ali]

Hey, @angelcaru can you review this??

[angelcaru]

To avoid that funny-looking code, we could have a special method on SymbolTables like set_builtin or something and have this function look for that. Otherwise, LGTM