prev => leftward, next => rightward

except in certain assert msgs 'prev' => '[L]' and 'next' => '[R]',
  in SupSub::respace, 'prevWidth' => 'leftWidth',
  and in TextBlock's logic to split apart,
    'prev{Block,Pc}' => 'left{Block,Pc}'

manual cherry-pick of 417b28d48e7dcf21610addd078e90f826d150d46
insertAdjacent => insDirOf

sed -i '' 's/insertAdjacent/insDirOf/g' src/*.js

manual cherry-pick of cc2b036152b59fa6e66549b3bb8b41583212577b
appendDir => insAtDirEnd

sed -i '' 's/appendDir/insAtDirEnd/g' src/*.js

manual cherry-pick of 9b120c4cd20d3243a25ee4fea65c4b15f47e52ce
"insert{Before,After}" => "ins{Left,Right}Of"

except gotta be careful not to replace the instances of calling
jQuery.fn.insert{Before,After}

manual cherry-pick of 582f175c88b22d36dd9f0a7cba6dd81b4cb1122f
"{pre,ap}pendTo" => "insAt{Left,Right}End"

except gotta be careful not to replace the instances of calling
jQuery.fn.{pre,ap}pendTo

also in moveUpDown comments, have to add 'of' for grammaticalness

manual cherry-pick of 5d9382718c75eb93bdb75e63daca39ae74b65122
reword comments/assert msgs to not say pre/append

cherry-pick of 79133d63cd40899107c97e57d3d591563440ab6c
TextPiece::{appendTextInDir => insTextAtDirEnd}
$ 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.

cherry-pick of d5597e4ab57a9ba6e6c232b69ff183126cfcb1be

Conflicts:
	src/cursor.js
	vendor/pjs
createBefore => createLeftOf

sed -i  '' 's/createBefore/createLeftOf/g' src/*.js

manual cherry-pick of b25406f4dd85b5e24cabcbe474a1918de9162ef3
Merge branch 'refactor.directions' into dev

a huge manual rebase of #160 onto dev