Adding Steven Levithan's String.prototype.split for RegExp

I asked and received his blessing:

> Tyler, thanks for offering to take this up! I’d be more than happy to see my code for this shared more broadly, and/or improved. My split shim and tests are MIT licensed, like es5 shim. By the way, Kris (author of es5-shim) brought this up before, but no one has done the work to see this through, like you’ve offered to do. See https://github.com/kriskowal/es5 shim/issues/103
> 
> /Steven

This also closes issue #103

Please let me know if there is anything I should do to change the code to follow some style guidelines.  I didn't see anything published anywhere, but I'll happily take feedback.