Escape </script and <!-- in inlined <script>s.

This is necessary to be able to use Polymer 0.5.1 with emcee.

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) and `<!--`, using the same tricks as [Polymer vulcanize](https://github.com/Polymer/vulcanize/)

I look forward to your feedback!