Move calculateFormula() into its own module.
Separate DOM manipulation into separate fns.
Whitespace.
Move operator functions onto operators obj.
Rename interpret() to toReversePolishNotation().
Make toReversePolishNotation() use locals.

* Do not store stack and outputQueue on interpreter obj.
Make cleanString() use locals.
calculate() uses passed outputQueue, not obj version.
Make go() use returned values from called fns.
Interpreter no longer needs to take str.
Remove state from Interpreter.
Pass pattern to go().
Make RPN get set by gui module.
Add test file.
Make gui.js use 4 spaces for a tab.
Remove unneeded var.
Rename cleanString() to tokenize().
Make calculate recursive.

* Get rid of unnecessary parseRPN inner fn.
Style: line breaks.
Can return right away from fn.

* No need to reassign tokens.