Fix multiline comment start

Over in #24, @mscheibe noted that:
> Compass Sprite imports can be of the form @import "folder/*.png". This matches the regex for multi-line comments and the rest of the file would then be ignored.

Looking at this more closely, I see that other, similar strings can cause the same failure:

- .rule { content: "/*"; }
- /* A multi-line comment on a single line */

Properly parsing CSS strings and comments is a difficult task. Fortunately, the KSS spec implies that we only should care about clean multi-line comments that start and end on their own lines. This is much easier to match with regex.