API: add subplot_kws to FacetGrid

Fixes #311.

Here is my example of an actual use case, for making maps with cartopy:
```python
%matplotlib inline

import numpy as np
import pandas as pd
import seaborn as sns
import matplotlib.pyplot as plt

import cartopy.crs
import cartopy.feature

rs = np.random.RandomState(1000)

lat = -90 + 180 * rs.rand(700)
lon = 360 * rs.rand(700)
cat = ['A'] * 100 + ['B'] * 200 + ['C'] * 400 

df = pd.DataFrame({'lat': lat, 'lon': lon, 'cat': cat})

g = sns.FacetGrid(df, col="cat", hue="cat", subplot_kws=dict(projection=cartopy.crs.Orthographic()))
g.map(plt.scatter, "lon", "lat", s=5, transform=cartopy.crs.PlateCarree())
for ax in g.axes.ravel():
    ax.add_feature(cartopy.feature.COASTLINE)
    ax.set_global()
```
![image](https://cloud.githubusercontent.com/assets/1217238/4517761/bca01544-4c53-11e4-8c0f-d111961b4185.png)

Maybe we should add an example like this to the documentation?