- don't use float.Epsilon since it does evaluate to 0 on ARM

Something the bit me recently: in an ActionInterval I got an Update call with NaN on the device on iOS. In the simulator it was working fine.

The problem is the use of float.Epsilon which is supported on ARM.

see here: http://msdn.microsoft.com/en-us/library/system.single.epsilon(v=vs.110).aspx
and here: https://bugzilla.xamarin.com/show_bug.cgi?id=15802