Adding `interactplot` function

The `interactplot` function is being added to help visualize models with two-way interactions between continuous variables. The plot is made by fitting an interaction model and then drawing a contour plot with the two predictor variables on each axis and the fitted values (y hats) color-encoded in the contours. So the joint distribution of the data observations is clear, a scatterplot is then drawn over the contours. The resulting image looks like this:

![screen shot 2013-12-10 at 11 18 39 pm](https://f.cloud.github.com/assets/315810/1721953/f32a2c80-6234-11e3-8cf3-c3125e0cf417.png)

This PR has the new function and some additions to the linear models notebook (including an example for an alternative presentation that bins one of the variables).