add hidePassively attribute for MMAlertView

[litt1e-p]

hi, @adad184 . thanks a lot for your great MMPopupView and I using it in my project. It works everything fine now. But sometimes we want to keep alertView no hiding when click items such as showing an app version update alertView that we look forward to making user to update app forcibly or some where we want to hide the alertView by ourselves. So I add an attribute named hidePassively to MMAlertView, the default value is NO which wil not affect the normal usage and it will keep showing the alertView when set to YES until calling the function: hide(). you can check the detail in my PR. May I make this PR to you? thanks for your MMPopupView again