Kill jQuery plugin; global MathQuill() returns API obj #big

Usage:

To create a MathQuill thing, just call MathQuill.StaticMath(),
.MathField() or .TextField() with an HTML element:

<span id="math-field"></span>

var mathField = MathQuill.MathField($('#math-field')[0]);
assert(mathField instanceof MathQuill.MathField);
assert(mathField instanceof MathQuill.EditableField);
assert(mathField instanceof MathQuill);

All of the old .mathquill('methodName', ...) methods are available on
this MathQuill object as .methodName(...) methods, plus a new .el()
method that returns the HTML element:

mathField.latex('x^2 + y^2 = 1');
$(mathField.el()).detach().appendTo(somewhereElse);
mathField.redraw();

The MathQuill global API object is also a function that you can use to
get the API object for an HTML element:

MathQuill($('#math-field')[0]).latex('y = \\pm \\sqrt{ 1 - x^2 }');
assert(MathQuill($('#math-field')[0]) === mathField);

---

For now I kept all the old, tired 'mathquill-embedded-latex',
'mathquill-editable', 'mathquill-textbox' CSS class names, but it'd be
nice to use more sensical 'mathquill-static-math',
'mathquill-math-field', 'mathquill-text-field' names instead.
README documents Usage of new API

But not MathQuill.TextField, because it's still a shitshow (paste not
working, etc).
Use syntax-highlighted code blocks in README
Fix Node.js link in README
Merge pull request #238 from mathquill/api.non-plugin

Kill jQuery plugin; global MathQuill() returns API obj #big