Add compatibility wrapper for getComputedStyle in IE8

[Update: this fixes the issue for IE8, but IE9 should have been OK to begin with]

Fixes #196.

The wrapper function is still significantly faster than using jQuery's `.css()` function, which may be important since this is called on every shiny element to see whether it is hidden:
  http://jsperf.com/jquery-css-vs-getcomputedstyle/2