Add hue_kws and markers to axis grids

This lets the user map the hue variable with arbitrary additional plot
aesthetics in `FacetGrid`/`PairPlot`, and it provides an easy interface to map the
scatterplot marker when using `lmplot`/`pairplot`.

```python
sns.lmplot("total_bill", "tip", tips, hue="smoker", markers=["x", "o"], scatter_kws={"s": 50})
```

![markers](https://cloud.githubusercontent.com/assets/315810/4519351/8e38aa2c-4cb7-11e4-9747-8d7d8f89379b.png)

```python
g = sns.FacetGrid(attention, col="subject", col_wrap=5,
                  hue="attention", hue_kws={"marker": ["s", "D"]},
                  aspect=.75)
g.map(plt.plot, "solutions", "score", ms=10, lw=3)
g.set(xlim=(.5, 3.5), xticks=[1, 2, 3], ylim=[0, 10])
```

![facetgrid](https://cloud.githubusercontent.com/assets/315810/4519652/fa62322e-4cc4-11e4-9207-64d1493df29f.png)

```python
sns.pairplot(iris, "species", markers=["o", "D", "s"], size=2)
```

![pairgrid](https://cloud.githubusercontent.com/assets/315810/4519477/f685ea62-4cbd-11e4-9595-e357936fc213.png)


This also has a small enhancement to change the marker params depending on the
kind of marker so that line-based glyphs are not invisible (which would
otherwise be the case with the default seaborn rc Params). This is only in `lmplot`, pending the introduction of `sns.scatterplot`.

Closes #310