Add residplot function

This is a pretty simple function for calculating and plotting the residuals of a linear regression:

``` python
mean = [0, 0]
cov = [(1, .5), (.5, 1)]
x, y = np.random.multivariate_normal(mean, cov, 100).T
sns.residplot(x, y, lowess=True)
```

![residplot](https://f.cloud.github.com/assets/315810/2416690/09b8a4cc-ab0a-11e3-8014-e4318bf97166.png)

Hopefully it makes life better for everyone.