Tweaked CSS URL regex to deal with empty url() properties and long data URIs

[DoomTay]

This pull request fixes #374 along with long data URIs containing parenthesis being misparsed, like here

I remembered to:

• Update or add unit tests if needed
• Update or add documentation/comments if needed
• Made sure stray files or whitespace didn't get committed
• Read the guidelines for contributing

Changes:

• Replaced {1,500} with an asterisk. This helps account for empty url() paths as well as longer data URIs that happen to have parentheses in them