adding covariance to dataframe

here is a sample implementation for dataframe.

``` python
def cov(self):
    """
    Compute pairwise covariance of columns

    Returns
    -------
    y : DataFrame
    """

    cols = self.columns
    std = np.diag(self.std())
    corr = self.corr()
    result = np.dot(np.dot(std, corr), std)
    return self._constructor(result , index=cols, columns=cols)
```
