Added Makefile to WebDriver spec

Hi,

I noticed that I was doing more and more hand-hacking to build the spec, so
put together a quick Makefile to reduce the tedium. To rebuild
webdriver-spec.html, just run "make all". Since "all" is the default
target, "make" will do.

I've only tested this on my OS X machine, so feel free to send a path to
make it work on other OSs.

Simon