Added legend support to kdeplot()

I've added a legend keyword to kdeplot() so that users can toggle legends on or off when displaying, e.g., multiple densities on the same plot. Labels do not override the legend setting, so you need to assign labels and set legend to "True" in order to see the legend as expected.

![pta_sce_with_legend](https://f.cloud.github.com/assets/369992/1694332/74131378-5e9f-11e3-9a3b-b762b76d26df.png)

This plot was created by modifying one of the examples from the IPython notebooks to read:
```
sns.kdeplot(nm_gen0, shade=True, color=c1, ax=ax1, label='AA', legend=True)
sns.kdeplot(nm_gen1, shade=True, color=c2, ax=ax1, label='Aa', legend=True)
sns.kdeplot(nm_gen2, shade=True, color=c3, ax=ax1, label='aa', legend=True)
```