Show Textareas button selected with id not tag name

want other buttons!

(cherry picked from commit aec8ed02dae67bd14d907fd5720ef6c6dd73d536)
Make visual tests' "Show Textareas" way less hacky #test

Rather than swapping the CSS class of all textareas, and then having to
monkey-patch jQuery.fn.prepend so that when static math dynamically adds
textareas, their CSS class gets swapped too, just override the CSS of
textareas.
Fix selecting already selected static math

Only prepend textareaSpan if not already inserted, otherwise will detach
and re-attach, synchronously firing a blur event which in an immediately
subsequent event thread detaches the textareaSpan.

Deals with #216 for non-IE8; relies `return false` in mousedown handler
preventing textarea from being blurred, though, which apparently doesn't
work in IE<9.
Merge pull request #230 from mathquill/fix.select-static-math

Fix selecting already selected static math
Merge branch 'master' into dev

Conflicts:
	src/rootelements.js
	- 138f279 on #230 fix.select-static-math on master changed when
	  we prepend the textareaSpan
	- e23f3de on #195 perf.latex-render on dev changed from
	  prepending textareaSpan to jQ, to prepending to container
	test/visual.html
	- git didn't notice that b5a4158 on #230 was a cherry-pick of
	  aec8ed0 on #195, so when e478710 built on top of the
	  cherry-pick it conflicted