BUG: rolling_max with center=True and min_periods=1 returns incorrect results
rolling_max with center=True and min_periods=1 returns NaN's 

```
Python 3.3.5 (default, Mar 10 2014, 03:21:31) 
[GCC 4.8.2 20140206 (prerelease)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import pandas as pd
>>> df=pd.DataFrame([0,1,2,1,0], columns=['a']

### This is OK
>>> pd.rolling_max(df['a'], window=3, center=True)
0   NaN
1     2
2     2
3     2
4   NaN
dtype: float64


### This should return [1, 2, 2, 2, 1]
>>> pd.rolling_max(df['a'], window=3, center=True, min_periods=1)
0     1
1     2
2     2
3     2
4   NaN
dtype: float64

>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.3.5.final.0
python-bits: 64
OS: Linux
OS-release: 3.13.6-1-ARCH
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.13.1
Cython: 0.20.1
numpy: 1.8.1
scipy: 0.13.3
statsmodels: 0.5.0
IPython: 1.2.1
sphinx: 1.2.2
patsy: None
scikits.timeseries: None
dateutil: 2.2
pytz: 2014.1
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.3.1
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
sqlalchemy: None
lxml: None
bs4: None
html5lib: None
bq: None
apiclient: None

```
