Close button, clear for filter search and new styles

Hi Nicolas,

I added a close button for the FilterBox and if the user clicks outside of the div it closes as well. Commit: 312e0f9
I believe you have an issue for this.

Also there is a clear for the search filter that appears once the user has started filling out the the search box.

Another new feature is text that tells you how many checkboxs have been selected.

I added a file called pivot_mod.css for new styles. For our site we are combining them but I kept them separate in case you don't need them.

Feel free to use some or all of these changes. I'm doing this work for Marius at Metric Insites.
Thanks again for all your hard work on Pivottable

best,
John