Discrete distributions

Starting work on discrete distributions for sympy.stats.  Note that the probability space / random variable infrastructure isn't fully set up.  You can create discrete RVs and ask for expectations and variances of expressions.  Conditional and product expressions are not supported.

Discrete random variables depend on SymPy `summation`s

```Python
In [1]: from sympy.stats import Poisson, density, E, variance

In [2]: from sympy.abc import z

In [3]: rate = Symbol('lambda', positive=True)

In [4]: X = Poisson('x', rate)

In [5]: density(X)(z)
Out[5]: 
 z  -λ
λ ⋅ℯ  
──────
  z!  

In [6]: E(X)
Out[6]: λ

In [7]: simplify(variance(2*X + 1))
Out[7]: 4⋅λ

In [8]: E(X**2 + 3*X, evaluate=False)
Out[8]: 
  ∞                  ∞            
 ____               ____          
 ╲                  ╲             
  ╲      x    -λ     ╲    x  2  -λ
   ╲  3⋅λ ⋅x⋅ℯ        ╲  λ ⋅x ⋅ℯ  
   ╱  ────────── +    ╱  ─────────
  ╱       x!         ╱       x!   
 ╱                  ╱             
 ‾‾‾‾               ‾‾‾‾          
x = 0              x = 0          
```