Workaround the device-width bug in Windows Phone 8 by specifying device-width for screens above 400px, and a fixed width of 320 CSS pixels if width is < 400 device pixels.

git-svn-id: https://plugins.svn.wordpress.org/mp6/trunk@800580 b8457f37-d9ea-0310-8a92-e5e31aec5664