PLT: add the ability to turn off the horizontal line at y = 0 generated by plot(kind='bar')
``` python
s = Series(np.random.choice(list('abcde'), size=(1000)))
ax = s.value_counts().plot(kind='bar')
ax.set_ylim(-10, 250)
plt.gcf().savefig('asdf.png')
```

![asdf](https://cloud.githubusercontent.com/assets/417981/2974726/95be81b4-db8c-11e3-82f5-2471aecadad6.png)
