MRG: fix zorder, hatch, and fill in root2matplotlib.hist

* always set zorder of the fill and edge line such that it is above the previous object and draw the line above the fill
* if the fill is hatched then don't fill it by default, to match ROOT's behaviour

Before:
![before](https://f.cloud.github.com/assets/202816/766890/b138a862-e870-11e2-8afd-aecb47d5bd5a.png)

After:
![after](https://f.cloud.github.com/assets/202816/766894/b675ebe6-e870-11e2-9d27-5736bc3ce158.png)

Equivalent in ROOT (see https://sft.its.cern.ch/jira/browse/ROOT-5241, ROOT does not draw stacks correctly):
![root](https://f.cloud.github.com/assets/202816/791857/e6d9b2f6-eb57-11e2-9d30-a63f3c309b55.png)


Source:
```python
from ROOT import gStyle, TColor
from rootpy.plotting import Hist, HistStack, F1, Canvas
from rootpy.plotting import root2matplotlib as rplt
from rootpy.interactive import wait
from matplotlib import pyplot as plt

gStyle.SetHatchesLineWidth(2)
gStyle.SetHatchesSpacing(2.5)
gStyle.SetOptStat(0)

c = Canvas()
histo = Hist(40, -4, 5, name='A1', drawstyle='hist')
histo.FillRandom('gaus', 5000)

color = TColor.GetColor('#e60077')
histo.SetLineColor(color)
histo.SetFillStyle('solid')
histo.SetFillColor(color)

histo1 = Hist(40, -4, 5, name='B1', drawstyle='hist')
histo2 = Hist(40, -4, 5, name='B2', drawstyle='hist')
histo3 = Hist(40, -4, 5, name='B3', drawstyle='hist')

f = F1('gaus', 0, 3)
f.SetParameters(10.,1.5, 1)

histo1.FillRandom(f, 1500)
histo2.FillRandom(f, 1000)
histo3.FillRandom(f, 2000)

color1 = TColor.GetColor('#ee7700')
color2 = TColor.GetColor('#0033cc')

histo1.SetLineColor(color1)
histo1.SetFillStyle('/')
histo1.SetFillColor(color1)
histo1.SetLineWidth(2)

histo2.SetLineColor(color2)
histo2.SetFillStyle('\\')
histo2.SetFillColor(color2)
histo2.SetLineWidth(2)

histo3.SetLineColor(color1)
histo3.SetFillStyle('/')
histo3.SetFillColor(color1)
histo3.SetLineWidth(2)

hstack = HistStack(name='hstackB')
hstack.Add(histo1)
hstack.Add(histo2)
hstack.Add(histo3)

ymax = max(histo.GetMaximum(), hstack.GetMaximum()) * 1.2
histo.SetMaximum(ymax)
hstack.SetMaximum(ymax)

histo.Draw()
hstack.Draw('SAME')

# draw with matplotlib
plt.figure()
axes = plt.gca()
rplt.hist(histo)
rplt.hist(hstack, stacked=True)
plt.show()

wait(True)
```