Made a css_html to obtain the html of the css element.

Suggested Reviewers: @wedaly @jzoldak 
I noticed that in the two recent builds of acceptance tests, two of the steps were flakey due to trying to access some part of the element and getting a stale reference exception.  Since both steps were accessing the same parts of the element, I decided to make a function to access it.

This works like css_click in that it will retry if there was a stale element reference exception.  This was tested on both mac and ubuntu and it seemed to work.