Add Closure support

How to use:
Run setup-closure.sh from the same dir as apprtc.py. This will download the closure library and compiler.
Run closure-build.sh to compile the files in closure-src. The output is in closure-out as compile.js, which can be referenced from the HTML file in a script tag.

closure-src/ only has a utils.js for now. We can convert the other pieces gradually.

@phoglund