Added an option for pivotTableRenderer for being able to choose
what kind of content th must render, thContent: text/html.
If someone needs to add images or other html to th it will be able to.
All started from this SO question: http://stackoverflow.com/q/26002789/832363
I also created an example: examples/simple_ui_th_html.html in order to show
how this option can be used.