Make MultiDiscrete a Tuple-like space

[XuehaiPan]

Add __getitem__ and __len__ method for MultiDiscrete space.

As exampled in Tuple:

gym/gym/spaces/tuple.py

Lines 5 to 11 in 2bd436b

	class Tuple(Space):
	"""
	A tuple (i.e., product) of simpler spaces
	Example usage:
	self.observation_space = spaces.Tuple((spaces.Discrete(2), spaces.Discrete(3)))
	"""

this PR makes:

t = Tuple((Discrete(2), Discrete(3)))
md = MulitDiscrete([2, 3])

t[0]   # -> Discrete(2)
md[0]  # -> Discrete(2)

[jkterry1]

@XuehaiPan Can you please add tests?

[XuehaiPan]

@XuehaiPan Can you please add tests?

Done.