Accepts CoffeeScript requires with spaces. Fixes #57.

Updated RequireReference regex to match either an opening parenthesis or
space after the "require" keyword, and made the closing parenthesis
optional. These changes allows valid CoffeeScript syntax to be accepted. I also made the double/single quotations mandatory since I think they're expected for this type of reference. Keeping them optional caused the path group to incorrectly match one character (not the entire file) and the way around that would've been to change the path group's pattern from `.+?` to `[^""']+`.