Simplify list of trig function names

(AMS-?)LaTeX itself actually supports only the 'arc-' prefix, and only
on trigonometric and not hyperbolic functions, and only a limited subset
at that:
(search "Trigonometric Functions")
http://en.wikibooks.org/wiki/LaTeX/Mathematics#List_of_Mathematical_Symbols

In fact, on hyperbolic functions the prefix is supposed to be 'ar-', as
in 'arsinh', 'arcosh', etc:
https://en.wikipedia.org/wiki/Inverse_hyperbolic_function#Logarithmic_representation
Auto-unitalicize sin, log etc as seq of Variable's #big

Rather than each being a single NonItalicizedFunction command like \sin,
they're now a sequence of individual letters (Variable's) with special
styling. Any sequence of Variable's 's', 'i', and 'n' will be
automatically unitalicized to look like \sin, but the cursor can go
between and edit individual letters.

Node::respace is the only hook called when a sibling, rather than a
descendant, is inserted or deleted, so Variable::respace is where we
have to search for the relevant contiguous run of letters and
unitalicize any substrings therein.

Also add an UnItalicized class so that typing or parsing \sin, \cos,
\log etc results in the appropriate sequence of Variable's.
Fix .writeLatex() forgot to .respace() siblings #bugfix

also LatexFragment::createLeftOf, which was largely copied from it
.sibling{Created,Deleted}() supersedes .respace() #rearchitect #doTodo

.respace() was the only hook called on a very useful event, the creation
or deletion of an immediate sibling, but wasn't provided enough
information. Replace .respace() with hooks with specific information
about the creation or deletion of immediate siblings.

This lets Variable, which wants to auto-unitalicize contiguous runs of
letters whenever they're edited, to only search for contiguous runs once
per insertion or deletion of a letter, rather than each letter
.respace() is called on (that's 3 letters for a letter inserted between
2 other letters) repeating the same search.

Note that neither .siblingCreated() nor .siblingDeleted() are called on
creation/insertion, but .contactWeld() is and the .jQ and siblings are
guaranteed to have been set up. (Is this abuse? I was originally
thinking .finalizeTree() but latex.test.js needs to call .finalizeTree()
for .latex() on SupSub's to work, but .jQ isn't set up so
PlusMinus::finalizeTree() blows up.)

Finally, get rid of Node::respace = noop, all the places where it saves
having to do feature detection, have to do a null check anyway.
Get rid of unitalicized fns containing 'f'

They're unimportant, current LatexCmds.f doesn't work with
auto-unitalicization because it descends from Symbol but not Variable,
and it's a hassle to get it to work because the way the visual
unitalicization works is to change the font from Times New Roman to
Symbola, but unlike Roman letters the florin character is still in
italic style, so 'hcf' looks like an unitalicized 'hc' but an italicized
'f'.

This should go away when MathQuill finally switches to STIX fonts, or
when I finally ragedelete LatexCmds.f.
Add and use Letter that descends from Variable

Only Letter, not other Variable's, have all the auto-unitalicization
shit. In particular, Letter::autoUnItalicize now no longer needs to
check `.ctrlSeq.length === 1` of itself or any other nodes, itself can
now only be a Letter, and on other nodes can just check
`instanceof Letter`.
Add missing unitalicized oper's supported by LaTeX

Except ones with over/under lines/arrows, and except the two-word ones
like \injlim and \projlim

http://latex.wikia.com/wiki/List_of_LaTeX_symbols#Named_operators:_sin.2C_cos.2C_etc.
Remove unitalicized oper's not supported by LaTeX

\proj, \span, and \lcm aren't in LaTeX at all.

\mod is apparently pretty special in LaTeX, seems to do stuff with
spacing too:
http://www.artofproblemsolving.com/Wiki/index.php/LaTeX:Commands#Mods
Sort unitalicized fns list alphabetically
Support 2-word unitalicized LaTeX operator names
Comment UnItalicizedCmds list with link #comment
Add spacing left of unitalicized operator names

Apparently they should have extra spacing on both sides:
http://tex.stackexchange.com/a/48463
Simplify respacing of last unitalicized letter
Merge pull request #247 from mathquill/feature.auto-unitalicize

Feature: Auto-unitalicize sin, log etc as seq of Letter's