Fixed zero height issue on pre-Honeycomb devices. Closes #66.

DefaultHeaderTransformer.getActionBarSize() returns 0 on pre-Honeycomb devices, because android.R.attr.actionBarSize is not used in ActionBarSherlockCompat. Instead the attribute com.actionbarsherlock.R.attr.actionBarSize sets the ActionBar's height.

So if android.R.attr.actionBarSize is not available, DefaultHeaderTransformer uses the height defined in the header layout as fallback. This fixes #66.