Improve the error message for an empty selector.
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.