Make the pmf's of discrete distributions symbolic

[pelegm]

For example, make the pmf of the binomial distribution be something like

def pmf(self, k):
    n, p = self.n, self.p
    return choose(n, k) * p ** k * (1 - p) ** (n - k)

A few things to note:

1. The choose function is called binomial in SymPy
2. We may need to sympify k before putting it inside the calculation (not sure, we may wish to verify this)