Fix #227 .less files not being auto-recompiled

Quick googling turns up hundreds of wrappers around fs.watch() that
watch recursively, as well as doing lots of unnecessary stuff like
allowing globbing paths, all of which are also hundreds of lines long.

The fs.readdir() trick is from isaacs' filewatcherthing:
https://github.com/isaacs/filewatcherthing/commit/740c76db4bcfd96bb087eca7f47d02828df2788d

I'm not sure why isaacs guarded against '.' and '..', fs.readdir()
supposedly (and empirically) excludes them:
http://nodejs.org/api/fs.html#fs_fs_readdir_path_callback
test_server.js: watcher updates dir's known files #ImAnIdiot

Rather than, files that are new in a directory since `make server` first
started, being rewatched every time any change happens in the directory.
Add BigSymbol Class to math/command
Remove BigSymbol::placeCursor #cleanup

I forgot to remove placeCursor when cherrypicking the desmos
customization.
https://github.com/mathquill/mathquill/pull/256#discussion_r10264620
Fix editing \int and move back to symbols.js

\int needs an html element
https://github.com/mathquill/mathquill/pull/256#discussion_r10244679
Cleanup setting BigSymbol up/down pointers

https://github.com/mathquill/mathquill/pull/256#discussion_r10264697
Use unicode char as key in LatexCmds
Change BigSymbol to SummationNotation

It's not really a symbol anymore.
Fix SummationNotation tests to use .write

https://github.com/mathquill/mathquill/pull/256/files#r10264955
Change BigSymbol to SummationNotation in CharCmds
Fix SummationNotation lower limit block spacing #visual

https://github.com/mathquill/mathquill/commit/18f5a67c5e165d96f15172a85cf97096c66f63f5#commitcomment-5705893
Merge pull request #258 from mathquill/feature.SummationNotation

Feature: SummationNotation
Merge pull request #228 from mathquill/build.watch-recursively

Fix #227 .less files not being auto-recompiled
Merge branch 'master' into dev
Merge pull request #259 from mathquill/fix.focus-ring

Fix blue focus ring to only be around whole field
Merge pull request #262 from laughinghan/feature.auto-expanding-parens

Feature: auto-expanding parens
Remove obsolete .addClass('hasCursor') #cleanup

Has been obsolete for 4 years, since [f44c868][] added a .focus() call
(then called .removeEmpty()) to Cursor::show, which calls
.addClass('hasCursor')[1], and which itself was called by the 'focus'
event handler on the textarea[2].

[f44c868]: f44c8689ac7b849c63cb4380d0d3978115365a27
[1]: https://github.com/mathquill/mathquill/blob/f44c8689ac7b849c63cb4380d0d3978115365a27/src/backend.js#L177
[2]: https://github.com/mathquill/mathquill/blob/f44c8689ac7b849c63cb4380d0d3978115365a27/src/frontend.js#L781
Handle intentional blur

That is, if the typist deliberately clicks outside an editable field,
the selection clears and any ghost parens solidify; but if the typist
just switches tabs/windows and switches back, nothing should change.

[QuirksMode][1] reports a few browser bugs with focus events on the
window but none with blur events on the window registered in the
bubbling phase.

[This widely cited blog post][2] agrees except that in IE6/7 (others
versions not mentioned), spurious blur events fire on the window "when
the focus went from the body to an input or link"; in other words, if
the page is in focus but no particular element is focused
(document.activeElement === document.body), and then something on that
page is focused (the page remains in focus, so sane browsers don't fire
blur on the window). Luckily that doesn't affect this code, we're not
listening for blur on the window except immediately after a blur on
MathQuill's hidden textarea.

[1]: http://www.quirksmode.org/dom/events/blurfocus.html
[2]: http://www.thefutureoftheweb.com/blog/detect-browser-window-focus
Merge branch 'feature.intentional-blur' into myfork.gh-pages