Fix helper/internalize, add a pure lua test, test some string functions

This functions as an example of what test writing in pure lua would be like. I've also taken the liberty of "testing" some functions. Perhaps I found one bug (the commented out one), would need someone else to confirm.

Putting it up here for some discussion. Any comments @tarruda?

**EDIT**: By the way, currently I run them with `busted --pattern=. ./test` because the extension is different of course.