Add property which allows header view to handle touch events.

Previously, the header view could not accept user interaction. By setting the `mAllowHeaderTouchEvents` flag, the margin view will pass touches to the header view to be (possibly) handled.

Common use cases are if the header view contains an ImageView which needs an [OnClickListener](http://developer.android.com/reference/android/view/View.OnClickListener.html) or if you have a more complex header view with buttons or other clickable elements.