csslint: Add .csslintignore file and update gruntfile

Add .csslintignore file for the csslint run by Jenkins.

Also updating our Gruntfile for local usage to include demos,
which Jenkins is going to include as well (as it uses an ignore
blacklist instead of whitelist).

Change-Id: I9114cfc54e82f090f0fcf62155ef7c1a9261548d
Merge "csslint: Add .csslintignore file and update gruntfile"