allow user to change default windowLevel used in SDContentAlertView

Multiple instances of UIWindow at the same windowLevel can cause issues with keyboard handling (and perhaps other things as well)

These changes allow the caller of SDContentAlertView to avoid this issue by specifying a different default windowLevel during app startup (actually, anytime before showing an alert).  If the default is not changed, it will use UIWindowLevelAlert as before. 

If no other class in the project uses secondary UIWindow instances, there is no need to change the value.