Bug in rolling_* functions when combining center and min_periods arguments
Hello,
I have found some incorrect behavior in the pd.rolling_\* functions regarding the combination of 'center' and 'min_periods' arguments.  I believe this is an issue for all rolling_\* functions.  Here is an example using rolling_mean:

When using only the 'center' argument, the behavior works as expected:

``` python
In [5]: df = pd.DataFrame(range(10)) 
In [6]: df
Out[6]: 
   0
0  0
1  1
2  2
3  3
4  4
5  5
6  6
7  7
8  8
9  9
In [7]: pd.rolling_mean(df, window=3, center=True)
Out[7]: 
    0
0 NaN
1   1
2   2
3   3
4   4
5   5
6   6
7   7
8   8
9 NaN
```

When using the 'min_periods' argument, the behavior also works as expected:

``` python
In [8]: pd.rolling_mean(df, window=3, min_periods=1)
Out[8]: 
     0
0  0.0
1  0.5
2  1.0
3  2.0
4  3.0
5  4.0
6  5.0
7  6.0
8  7.0
9  8.0
```

When combining both the 'center' and 'min_periods' arguments, however, the result is incorrect.  The last entry should be 8.5 I believe.

``` python
In [9]: pd.rolling_mean(df, window=3, center=True, min_periods=1)
Out[9]: 
     0
0  0.5
1  1.0
2  2.0
3  3.0
4  4.0
5  5.0
6  6.0
7  7.0
8  8.0
9  NaN
```

``` python
In [10]: pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.3.final.0
python-bits: 64
OS: Linux
OS-release: 3.2.0-4-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.13.1
Cython: 0.19.2
numpy: 1.8.0
scipy: 0.13.0
statsmodels: 0.6.0.dev-bed3499
IPython: 2.0.0
sphinx: 1.2.1
patsy: 0.2.1
scikits.timeseries: None
dateutil: 1.5
pytz: 2012c
bottleneck: None
tables: 2.3.1
numexpr: 2.0.1
matplotlib: 1.3.1
openpyxl: 1.5.8
xlrd: 0.6.1
xlwt: 0.7.4
xlsxwriter: None
sqlalchemy: None
lxml: None
bs4: None
html5lib: None
bq: None
apiclient: None
```
