Fix exception on bad directive '@'

Without this fix the following exception was raised:
NoMethodError: undefined method `gsub' for nil:NilClass
Merge pull request #1063 from felixbuenemann/fix-empty-directive-exception

Fix exception on bad directive '@'
Old link for IE Filters dead - added new link.

I think this is close to what the old link had; though microsoft.com suggests http://msdn.microsoft.com/en-us/library/ie/aa740473.aspx.
Merge pull request #1079 from KatieK2/patch-1

Old link for IE Filters dead - added new link.
Merge branch 'stable'

Conflicts:
	lib/sass/engine.rb