FEAT: Add jitted function for drawing

[QBatista]

Adds a jitted function for drawing discrete random variables. Note that DiscreteRV.draw no longer uses check_random_state as Numba supports top-level functions from the numpy.random module, but does not allow you to create individual RandomState instances.

Close #390

[oyamad]

Some comments:

• Why not keep the DiscreteRV class as is, just adding the new jitted discrete_rv function? A DiscreteRV instance cannot be passed to a jitted function anyway (unless it is implemented as a jitclass).

• To me, discrete_rv sounds like returning some object representing a discrete random variable. What about random_choice? Note that this is similar to np.random.choice, where Numba does not support the p option (see this).

[oyamad]

The default value for seed must be None.

[QBatista]

I agree with those comments. @jstac What do you think?

[coveralls]

Coverage decreased (-0.02%) to 94.893% when pulling 081163f on QBatista:jitted_discreterv into 49ad385 on QuantEcon:master.

[jstac]

@QBatista Many thanks.
@mmcky Do you mind if we merge this now? It looks good for me and I need it to finish what I'm doing in the master branch of the lectures...

[jstac]

@mmcky I'm going to go ahead and merge this, i hope that's OK.