slimerjs: added xulrunner dependency

**Original PR without mavericks hacks: https://gist.github.com/chrmoritz/7144640**

This is an alternative pull request for #22914 , which adds xulrunner as a resource to the slimerjs formula.

The problem with slimerjs just depending on xulrunner as in #22914 is, that xulrunner is designed to generate a framework on os x which can be used by 3rd party apps. We can't symlink the xulrunner executable into bin (which would be enough to be recognized by slimerjs), since xulrunner assumes, that all dylibs are in the same folder as the executable, which is not the case if you run it through the symlink (Couldn't load XPCOM error).

This PR adds xulrunner to the slimerjs files in the same way as in the [slimerjs standalone edtion](http://www.slimerjs.org/download.html#standalone) with the difference, that we compile xulrunner from source instead of shipping precompiled binaries from mozilla.

Edit: added simple loadspeed script to tests
Edit2: changed to use homebrew's curl instead of system 'curl ...'