Try to work around </script> in inlined <script>s.

Scripts that contain the string "</script" close the <script> tag when
inlined. This occurs in real code (e.g., in polymer 0.5.1), so it needs
to be worked around.

This change escapes scripts that contain the string "</script" (and
variations w/ whitespace), by wrapping the whole script in HTML
comments <!-- -->.

At this time, if a script contains both "</script" and "-->", we don't
know how to escape it, so we throw an exception. Hopefully, this is
better than having to debug this in Chrome later.