added von_mises_rng with error checks

still need to add goodness of fit test
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
Merge branch 'develop' into feature/vm_rng
fixed von_mises_rng and added chi square goodness of fit test
added von_mises rng to function signatures with test
added doc
updated range of return values for von_mises_rng
fixed von miss rng and added more tests
updated range of vm_rng in manual
added copyright information
fixed indexing in von_mises chi square test
Merge pull request #704 from stan-dev/feature/vm_rng

Fixes #647. Feature/vm rng