Disabled global HTML escaping, while always escaping within code blocks.

This follows from https://github.com/evilstreak/markdown-js/pull/34, opting to disable HTML escaping except in code blocks. I understand that it's been stated to be undesirable by the maintainers, but I think it is a more consistent implementation of Markdown.

I've added a test that mimics the handling here http://babelmark.bobtfish.net/?markdown=%3Cp%3EHTML%3C%2Fp%3E%0D%0A%0D%0A++++%3Cp%3ECode%3C%2Fp%3E&src=1&dest=2