Add xkcd color dictionary and palette function

Matplotlib can accept a lot of named colors, but haven't you ever wanted to make a plot with "greyish blue", "terracotta", and "dusty green"? Thanks to the [xkcd color survey](http://xkcd.com/color/rgb/) now you can:

```python
colors = ["butter yellow", "windows blue", "faded green", "greyish", "dusty purple"]
sns.palplot(sns.xkcd_palette(colors))
```
![screenshot 2014-06-25 18 29 12](https://cloud.githubusercontent.com/assets/315810/3393900/50fe68ee-fcd1-11e3-867f-75d3d796fa81.png)

```python
plt.plot(x, y, sns.xkcd_rgb["soft green"], lw=3);
```
![screenshot 2014-06-25 18 29 47](https://cloud.githubusercontent.com/assets/315810/3393903/739f4f12-fcd1-11e3-9303-485d39ca069b.png)