Fix some warnings.

Closes #906
Avoid another CSS counters bug on IE7.

Closes #908
Merge branch 'stable'

Conflicts:
	lib/sass/script/functions.rb
	lib/sass/tree/visitors/cssize.rb
Ensure that the static selector regex doesn't match empty selectors.

This makes the #selector_sequence method properly return nil if it
can't parse a selector, even if that selector precedes a bracket.
Fix the output style for unknown directives.
Add an @at-root directive.
Add a script node for accessing the current selector.
Add docs for @at-root and script &.
Refactor Sass::Engine#parse_directive.

Split the case statement into one method for each directive.