Added correlation(), moment(), cmoment() and smoment() in rv_interface.py

Added `correlation()`, `moment()`, `cmoment()` (central moment), smoment() (standardized moment) and examples for `skewness()` in `rv_interface.py`. 

    >>> from sympy.stats import Die, moment, E
    >>> X = Die('X', 6)
    >>> moment(X, 1, 6)
    -5/2
    >>> moment(X, 2)
    91/6
    >>> moment(X, 1) == E(X)
    True

    >>> from sympy.stats import Die, cmoment, variance
    >>> X = Die('X', 6)
    >>> cmoment(X, 3)
    0
    >>> cmoment(X, 2)
    35/12
    >>> cmoment(X, 2) == variance(X)
    True

    >>> from sympy.stats import Exponential, correlation
    >>> from sympy import Symbol
    >>> rate = Symbol('lambda', positive=True, real=True, bounded = True)
    >>> X = Exponential('X', rate)
    >>> Y = Exponential('Y', rate)
    >>> correlation(X, X)
    1
    >>> correlation(X, Y)
    0`