Added sinH function to FlxMath

Might be nice to add others...
The `h` doesn't seem to be capizalized generally.

- http://msdn.microsoft.com/en-us/library/system.math.sinh(v=vs.110).aspx
- https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/sinh
- http://www.tutorialspoint.com/java/lang/math_sinh.htm

Let's stick to that convention.