attempt at new API with global MathQuill function

Usage:

To create a MathQuill thing, must call `MathQuill.StaticMath`, `.MathField`
or `.TextField` and pass in something that jQuery resolves to a single
element.
```html
<span id="editable-math"></span>
```
```js
var editableMath = MathQuill.MathField('#editable-math');
assert(editableMath instanceof MathQuill.MathField);
assert(editableMath instanceof MathQuill);
```
All of the old `.mathquill('methodName', ...)` methods are available on
this MathQuill object.
```js
editableMath.latex('x^2 + y^2 = 1');
$('#editable-math').detach().insertAfter(somewhereElse);
editableMath.redraw();
```
The global MathQuill API object is also a function that you can use to
get the MathQuill object of an HTML element.
```js
MathQuill('#editable-math').latex('y = \\pm \\sqrt{ 1 - x^2 }');
assert(MathQuill('#editable-math') === editableMath);
```
---

I made minimal changes to `test/{demo,visual}.html` for all the tests to
still pass. This meant only deprecating rather than removing the old,
tired `mathquill-embedded-latex`, `mathquill-editable`,
`mathquill-textbox` CSS class names. Should those go next, or should
we keep those for backwards compatibility?
I am _so_ confused how it's possible that we **_both_** looked at this and were like "eh, don't see any way around this", when it's so obvious: ab4840c58984be13a1bf54c2e8432cbeaf9f34b3