Better attribute handling and some other minor changes

[mina86]

1. Most importantly, reorder attributes to avoid spaces.
2. There’s no need for \n after <!DOCTYPE…>.
3. Never turn boolean attributes into attributes with empty value.
4. Add cls attribute so it’s easier to extend HTMLMinParser.
5. Some refactoring to use io.StringIO rather than a list.

[mankyd]

FYI: I've merged this, plus a few small cleanups. Thanks a ton for this.