PairGrid and pairplot for drawing scatterplot (and more!) matrices

A new axisgrid object for drawing grids of paired plots. And a corresponding function for doing so easily.

Simple example:

```python
sns.pairplot(iris, hue="species")
```
![pairplot](https://cloud.githubusercontent.com/assets/315810/4071125/5134444e-2e6a-11e4-9ddc-8534c8cc5e80.png)

More complex example:

```python
g = sns.PairGrid(iris, diag_sharey=False)
g.map_lower(sns.kdeplot, cmap="Blues_d")
g.map_upper(plt.scatter)
g.map_diag(sns.kdeplot, lw=3)
```
![pairplot_kde](https://cloud.githubusercontent.com/assets/315810/4071130/688cbd7e-2e6a-11e4-868e-25632541b83e.png)


Remaining todos:
- [x] Add more tests
- [x] Update release docs
- [x] Rename `set_legend` to `add_legend` (and deal with in re: `FacetGrid`)