Using 'by' and 'weights' together with DataFrame.hist() results in ValueError: weights should have the same shape as x
Wanted to produce grouped histogram such that the heights of the bars add up to 1. The following code results in **ValueError: weights should have the same shape as x**

```
import pandas as pd
import numpy as np
import matplotlib.pyplot as plt

np.random.seed(123)

n = 100
df = pd.DataFrame(np.random.randn(n), columns=['a'])
by = np.random.randint(1,5,n)
df.hist(by=by) # works
plt.show()
weights = np.repeat(1/len(df), len(df))
df.hist(weights = weights) # works
plt.show()
df.hist(by = by, weights = weights) # does not work
plt.show()
```

In [15]: pd.show_versions()
## INSTALLED VERSIONS

commit: None
python: 3.4.2.final.0
python-bits: 64
OS: Linux
OS-release: 3.18.6-1-ARCH
machine: x86_64
processor:
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.15.2
nose: 1.3.4
Cython: None
numpy: 1.9.1
scipy: 0.15.1
statsmodels: 0.6.1
IPython: 2.4.1
sphinx: None
patsy: 0.3.0
dateutil: 2.4.0
pytz: 2014.10
bottleneck: 1.0.0
tables: None
numexpr: 2.4
matplotlib: 1.4.3
openpyxl: 1.8.6
xlrd: 0.9.3
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
rpy2: 2.5.6
sqlalchemy: None
pymysql: None
psycopg2: None
