Media query support

Similar to CSS, media query support in UISS allows you to have layouts
specific to various device configurations (e.g.: iPhone 4 vs 5). This
code is also extensible to allow for additional properties (e.g.:
device-version: 6, device-version: 7) in the future.

Since UISS files are evaluated once at runtime, media queries can only
be used to evaluate device-specific properties.

The syntax is a bit clunky, but a) it's understandable, and b) it's extensible.  I could see adding properties for device-version (iOS < 7, >= 7), idioms (pad, phone) as well as retina/non-retina so that you could distinguish between retina iPad and iPad mini (for slightly larger font sizes).