Support transparent status bar for Android

Continuation of discussion here: https://github.com/apache/cordova-plugin-statusbar/pull/22#issuecomment-76480150

Seems like Android officially endorses transparent status bar now:
http://android-developers.blogspot.com/2015/05/android-design-support-library.html

Example implementation:
https://github.com/ekuwang/cordova-plugin-statusbar

Would be nice to have this capability in the official plugin. Thanks!