Selection: Check for ancestor fieldsets when matching :enabled/:disabled

Fixes #174

This is almost half again as big as #244... I'm still not sure it's worth it, but in for a penny ⇒ in for a pound and we might as well get correct results on non-IE browsers too. @timmywil?

```
   raw     gz Compared to master @ 5b9cdd67aea236aaa55f632cc08d3f8676eeddb9
 +1287   +410 dist/sizzle.js
  +421   +129 dist/sizzle.min.js
```