$ is now a directionalized subclass of jQuery,

rather than just an alias. It's in tree.js, immediately after the
directions are defined.

All the directionalized jQuery calls in cursor.js are now way prettier.

For minification, `jQuery` rather than `$` is now closured in intro.js.
Of course, in publicapi.js, now the MathQuill plugin has to be attached
to `jQuery.fn` not `$.fn`.

Pjs had to be upgraded to v3, because jQuery internally calls
this.constructor() like a million times every time you so much as think
about sneezing at it.
Still some jQuery features don't work on $ (thoroughly documented in $'s
comment block), requiring a place in publicapi.js and textarea.test.js
each to use jQuery instead of $.

Other misc. change: textarea.js comes before tree.js and jQuery is
supposed to be its only dependency anyway, so.