Affects:
Choicegroups, option inputs, textline, and formula equation inputs

Change the `MathJax.Hub.Queue(initializeRequest)` to a simpler function call
(`initializeRequest.call(this)`). This was failing to give a proper context
to initializeRequest, and `this.value` was turning up as `undefined`.

Also add a fallback if we need to display some code before MathJax finishes
its original typesetting.

I was stubbing out `Queue` in my specs, so the tests had to be changed around
a little.

Named `FormulaEquationInput` (name up for debate)

- Based off ChemEqnIn
- Add FormulaEquationInput in inputtypes.py
- Add a call to a skeleton method for a preview

javascript:

- Queue up some MathJax
- Put some ordering on the AJAX requests: add a parameter when the request was started, when it returns check that it isn't outdated before displaying the preview
- Tests

Note: we moved the `jsinput` tests and DISABLED them, because they were causing the tests to fail.