Pandas groupby histogram doesn't do transparant bars (alpha kwarg)
I would expect the following code to produce a plot with transparent bars, but it doesn't.

``` python
import matplotlib.pyplot as plt
import pandas as pd
import numpy as np
plt.style.use('ggplot')

N = 1000

df = pd.DataFrame({
  'a': np.random.randint(0, 2, N),
  'b': np.random.random(N)
})

#df.groupby('a').b.hist(alpha=0.5)
df.groupby('a').b.plot(kind="hist", alpha=0.5)
plt.legend()
plt.show()
```

Neither the commented out `hist` command, nor the `plot` command works.

Version information from `pd.show_versions()`:
## INSTALLED VERSIONS

commit: None
python: 2.7.6.final.0
python-bits: 64
OS: Linux
OS-release: 3.13.0-46-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.16.2
nose: 1.3.7
Cython: 0.20.1post0
numpy: 1.9.2
scipy: 0.13.3
statsmodels: 0.5.0
IPython: 3.0.0
sphinx: None
patsy: 0.2.1
dateutil: 2.4.2
pytz: 2015.4
bottleneck: None
tables: 3.1.1
numexpr: 2.2.2
matplotlib: 1.4.3
openpyxl: 1.7.0
xlrd: 0.9.2
xlwt: 0.7.5
xlsxwriter: None
lxml: None
bs4: 4.2.1
html5lib: 0.999
httplib2: 0.8
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
