Added test form and modified engine.js

I have added a test.html file that gets displayed in the browser when the user navigates to http://localhost:16000/. The user then uses this test form to provide his query.

I've also modified the engine.js file to remove redundant code. The mml (MathML) being returned to the callback function, is extracted from the HTML element with id="renderedMML". Here we need not call the toMathML() function to extract the mml from the jax, as the engine has already extracted and stored the mml in the HTML node.