Html5

I fleshed out [aredridel's work](https://github.com/markaby/markaby/pull/26). 

- added doctype and meta chartype
- all the new html5 tags and attributes
- added a second kernel method 'mab5'
- html5 mode loses the xml-style closing slashes
- wrote some spec
- works with Sinatra
- haven't tested with Rails