fix fillstyle=None and respect legendstyle in root2matplotlib.hist

```python
from matplotlib import pyplot as plt
from rootpy.plotting import root2matplotlib as rplt
from rootpy.plotting import Hist

a = Hist(100, -5, 5, title='A')
b = a.Clone(title='B')

a.fillstyle = None
a.linewidth = 2
a.linecolor = 'red'
a.linestyle = 'dashed'
a.legendstyle = 'L'

b.fillstyle = 'solid'
b.fillcolor = 'blue'
b.linecolor = 'green'
b.linestyle = 'solid'
b.legendstyle = 'F'

a.FillRandom('gaus', 1000)
b.FillRandom('gaus', 1000)

plt.figure()
rplt.hist([a, b], stacked=True)
plt.legend()
plt.show()
```

![figure_1](https://f.cloud.github.com/assets/202816/679764/df4a5602-d979-11e2-82e6-44df51a3ecb8.png)