extract doctests from JavaScript block comments

Closes #27

This change proved significantly more involved than I first imagined, as it necessitated a second parsing phase.

While working on this change I realized that [escodegen][1] is not actually required: we needn't prettify code before testing it. :)

As always, I'd love to hear your feedback, @danse.


[1]: https://github.com/Constellation/escodegen