[imps] sniffing implementation

[ryan]

I'm just added the beginnings of a sniffing implementation to the  
ruby port of html5lib:

   http://html5lib.googlecode.com/svn/trunk/ruby/lib/html5/sniffer.rb

It only does HTML vs Feeds so far. More to come.

AFAICT, this might be the first implementation of this from the spec,  
so any feedback would be greatly appreciated.

Also, there are some tests at:

  http://html5lib.googlecode.com/svn/trunk/testdata/sniffer/

thanks,
ryan