option for ios7 header padding for statusbar overlay

Issue
-
For iOS7 style statusbar overlay when building Cordova app, there needs to be option to enable a 20px padding in header for iOS7 theme.

Fix
-
- Added a new afui property called `$.ui.overlayStatusbar`, which is false by default.
- If set to true, then it will check for `$.os.ios7` and then will add a class `overlayStatusbar` to `.header` elements.
- Defined `overlayStatusbar` class in `ios7.css` with `padding-top:20px` and `height:64px`
- Added this option in kitchensink `$.ui.overlayStatusbar = true`

![screen shot 2014-04-10 at 11 14 01 pm](https://cloud.githubusercontent.com/assets/1414842/2676808/6ee2de5a-c142-11e3-97f0-73e624ac6e8b.png)

Also fixed few minor issues in 2 more commits:
- default modal header had extra `.header` class
- sidemenu text color was wrong