Add support for pickle protocol 2 for Vec3

[rmcgibbo]

Currently, vec3 doesn't support the highest protocol for pickle. This PR adds support, via this magic method.

>>> # current master
>>> import pickle
>>> import simtk.openmm as mm
>>> vec3 = mm.Vec3(1,1,1)
>>> assert vec3 == pickle.loads(pickle.dumps(vec3, pickle.HIGHEST_PROTOCOL))
Traceback (most recent call last):
  File "f.py", line 5, in <module>
    assert vec3 == pickle.loads(pickle.dumps(vec3, pickle.HIGHEST_PROTOCOL))
  File "/home/rmcgibbo/opt/python/2.7.5/lib/python2.7/pickle.py", line 1382, in loads
    return Unpickler(file).load()
  File "/home/rmcgibbo/opt/python/2.7.5/lib/python2.7/pickle.py", line 858, in load
    dispatch[key](self)
  File "/home/rmcgibbo/opt/python/2.7.5/lib/python2.7/pickle.py", line 1083, in load_newobj
    obj = cls.__new__(cls, *args)
TypeError: __new__() takes exactly 4 arguments (2 given)