Cascade optimization issue

This speeds up cascade calculation tremendously if element names aren't heavily used in styles.

In my test case (a simple email campaign) with many styles but not many element name based styles) time to compute went from 1.851 seconds to 0.197 seconds.

In a much more complicated test case that used element name selectors heavily, time with this change went from 3.854 seconds to 3.92 seconds. On the average I think it's a huge improvement.

I'll look into further optimizations as I can.

The later test case is the following ridiculously long [HTML](http://s.amro.co/code/1h2h1J3Z1L3t). It was so long I couldn't embed it.