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
Merge pull request #165

Normalize string split
Normalize spaces, fix bug in new string split
Minify
2.1.0