xulrunner 26.0 + slimerjs 0.9 with xulrunner dependency

added new formula for Mozilla XULRunner (possible dependency for SlimerJS)

There are [xulrunners packages in macports](http://www.macports.org/ports.php?by=name&substr=xulrunner), but they are very outdated (xulrunner versions shiped with firefox 3.6). Since the current build system is completly different from the old one, this does not help much.

Xulrunners build instruction (very outdated): https://developer.mozilla.org/en-US/docs/XULRunner/Build_Instructions
general build instruction (mainly written for firefox): https://developer.mozilla.org/en-US/docs/Developer_Guide/Build_Instructions