Added synchronized scrolling for comparing taxa summary plots

Added support for synchronized scrolling of taxa summary plots when comparing self to other.

I put the following comment in the code, but it is worth reproducing here:

Synchronized scrolling has been tested in Chrome, Safari, and Firefox. It will work in all browsers when hosted (i.e. not opened locally). If opened locally, Chrome will not support synchronized scrolling because it does not allow javascript to access properties of the iframes on the local machine for security reasons. In this case, all other aspects of the page continue to function, but synchronized scrolling is disabled. This issue can be circumvented in Chrome by starting it with the ```--allow-file-access-from-files``` flag. Firefox and Safari support synchronized scrolling when run locally.

See http://stackoverflow.com/a/5664399 for more details.