BUG: fixed weird case where boxplot whiskers went inside box

Hey folks,

This PR takes care of a case where an oddly-distributed and small dataset causes some mayhem in the boxplots. An example that reproduces this error in the current `master` is below:

```python
import numpy as np
import matplotlib.pyplot as plt
x = np.array([3, 9000, 150, 88, 350, 200000, 1400, 960], dtype=np.float64)
fig, ax1 = plt.subplots()
ax1.boxplot(x)
ax1.set_yscale('log')
ax1.yaxis.grid(False, which='minor')
ax1.xaxis.grid(False)
plt.show()
```

...Which produces:
![bad-whisker](https://f.cloud.github.com/assets/1163939/300997/df0cd3ca-95b5-11e2-8464-fa62bae33664.png)

The changes to axes.boxplot where very minor and I've include an new test with baseline images as well. Thanks for the effort, y'all.