Fix some minor CSS issues with the expressions widget.

Summary:
Added a bit of vertical space between input and preview.
Made it so that only the MathJax will scroll if it overflows horizontally.
Made sure that error buddy + message fit within narrowest layout.
Set up testrender.html for testing in IE8 (using es5-shim.js).
Shrunk the error-buddy png to the size we use (from @2x).

Test Plan: Play around with the expression widget on testrender.html

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D2949