Switch to generating HTML ourselves

Switch to generating HTML ourselves, without the help of any XML/HTML libraries. This gives us much better direct control of HTML output, makes it much easier to provide consistency across Rubies, removes exposure to Nokogiri bugs, and furthers the goal of removing Nokogiri as a required dependency of Maruku.

@distler, I'm especially interested in how this works in your application, since you seem to really put Maruku through its paces and can probably find some problems that aren't yet in tests.