Add test case for total width of math fields

Specifically, a test case that 3f963a445a303b17699ad3d45f2b4f98105385c9
"Fix editable fields 6px wider than they should be" actually does what
it says it does, so setting the width of a math field correctly sets the
width, including any scrollbar, like other replaced elements like real
textareas.
Move border to container rather than root block

...so that the border can be styled (or removed) by code external to
MathQuill without having to know any internal CSS class names like
"mq-root-block".

(The box-shadow has to be on the container too because the root block is
inside and therefore on top of the container, so the box-shadow would
cover the changed border color. Even if it didn't, it makes the most
sense staying with the border.)

The padding absolutely has to be on span.mq-root-block, though, because
it's overflow:hidden, so without the extra padding inside its scroll
area, the cursor is hidden when at the left edge.

This does mean the dimensions are no longer like what I thought replaced
elements were like and uses the brain-dead Netscape box model, but it
turns out that at least textareas and inputs actually use the Netscape
box model anyway (in Chrome 34, at least): http://jsbin.com/qoceda/3/edit
Which makes sense since, like we want MathQuill math fields to be, their
border is configurable with external CSS. In fact, their padding is too,
so it actually kind of sucks that our padding can't because its not
the container that's overflow:hidden.

Recommend -w to view diff.
Combine .mq-root-block { ... } declaration blocks #simplify

Same selector, no reason they weren't combined in the first place.
Fix math field width test error message

https://github.com/mathquill/mathquill/commit/8b68c00f1bb7b066915343f409744236542134b7#diff-d5537ea249c99a3fcb91effad7262312L149
Merge pull request #340 from mathquill/feature.container-border

Move border to container rather than root block