Responsiveness

Hi, thanks for good piece of code!

When viewport's screen is narrower than 330px (and viewport scale is set to 1.0), dialog shows up not in the center of the screen and still has fixed width of 300px. I'd like to suggest to make it more resposive by setting width scalable when viewing on narrow devices.

EDIT: I've spotted #10 pull request. Merging would do the job I guess!
