Escape "class" attribute

Closes #1087

The fix was fairly straightforward: just required [an extra "if" clause](https://github.com/christiangenco/jade/commit/3274fb9ae67748b88189c79dd66bc0929d75ae6e) to test if the class attribute was escaped.

I also added a section in the `Readme.md` to help get started in contributing, as I hadn't used the mocha framework before and only figured it out after excessive googling and happening to google "mocha" after seeing the `test/mocha.opts` file.