Converting every dimension/length in all dialog styling to be in terms
of a 10px font size (chosen to be easy: 250px => 25em, 4px => .4em) makes
it easy to adjust the overall size of things by changing just that one
base font size unit that every dimension and length is in terms of.
For example, a one-line change of adjusting the base font size unit to
15px would scale everything up by a factor of 150%.

For the div, we can simply set the fontSize and convert all lengths from
px to em. Since the div is mostly just a container for the iframe, it
has few descendants and none override the fontSize (which is important
because em wouldn't be 10px on such a descendant).

For the splash page in the iframe, several elements do override the
font size, so instead we set the font-size of the root element ('html')
and convert all lengths from px to rem, aka "root em". Browser support
for rem is pretty good, no IE8 but IE9+ and all modern browsers:
  http://caniuse.com/rem
The one browser issue to deal with is that in IE9-10, rem doesn't work
in the 'font' CSS shorthand property (documented on "Can I use" and also
  http://stackoverflow.com/q/16157342/362030
and
  http://codersblock.com/blog/font-shorthand-bug-in-ie10/
) but the fix is pretty trivial.

Passed along from ?large to control the badge size, they now also
control the iframe and dialog size because after all, if you're on a
page with larger font-size that demands a larger badge, the small dialog
with small text is gonna look out of place:  https://git.io/v2Xy4

The large badge is about 1.5x the size of the small badge (30px to 20px;
the other ratios aren't as exact), but I found that making the large
dialog 1.4x the size of the small dialog looked better than 1.5x
(that is, bumping the em/rem unit font size from 10px to 14px rather
than 15px):

font-size: 14px  https://git.io/v2XDh
vs
font-size: 15px  https://git.io/v2XyJ

Conflicts with rauchg#153 feature.sign-in-link-in-iframe:
	lib/index.js
	  Added `large` param to splash() call, to which rauchg#153 had added `org`
	lib/splash.js
	  Converted px to rem where rauchg#153 had dedented