Update von_mises.hpp

Convention is that sign = 0 at 0
Change delta to s

Poor copy of original.
Ensuring range is 0 to 2pi 

I hope I used the correct syntax. Intent is to add 2pi and then take fmod of 2pi.
Only once times pi

Peter is right, need only addition of one pi to move into [0, 2pi]
Spoke too soon

Not working somehow; old way worked. Leave for now.
Marcus's suggestion & use TWO_PI

Implement Marcus's suggestion to ensure crazy mu's don't push answers out of the range and use TWO_PI since it is defined above already.
Merge pull request #658 from aadler/patch-1

Update Von Mises