Fixing warning message about UIViewAnimationOptions

Warning message solved :
MPNotificationView/MPNotificationView.m:421:33: Implicit conversion from enumeration type 'enum UIViewAnimationCurve' to different enumeration type 'UIViewAnimationOptions' (aka 'enum UIViewAnimationOptions')
Merge pull request #15 from pomarec/master

Fixing warning message about UIViewAnimationOptions