New index created by 2 weeks resampling is 1 week off
``` python
pd.Series( index=pd.date_range('2014-06-01', periods=20 ), data=range(20) ).resample( '2W-SAT', how='sum' )
Out[23]:
2014-06-07     21
2014-06-21    169
Freq: 2W-SAT, dtype: int64
```

The first entry of a 2 weeks resampling corresponds to the first week of the data.
The expected output should rather be 

``` python
2014-06-14    91
2014-06-28    99
```

The same problem appears if I start the Series on other Sundays: 

``` python
pd.Series( index=pd.date_range('2014-06-08', periods=20 ), data=range(20) ).resample( '2W-SAT', how='sum' )
Out[14]:
2014-06-14     21
2014-06-28    169
Freq: 2W-SAT, dtype: int64
```

Version

```
In [18]:

pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.6.final.0
python-bits: 64
OS: Darwin
OS-release: 13.2.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: C
LANG: None

pandas: 0.13.1
Cython: 0.20.1
numpy: 1.8.1
scipy: 0.14.0
statsmodels: None
IPython: 2.1.0
sphinx: None
patsy: None
scikits.timeseries: None
dateutil: 2.2
pytz: 2014.3
bottleneck: None
tables: 3.1.1
numexpr: 2.4
matplotlib: 1.3.1
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
sqlalchemy: None
lxml: None
bs4: None
html5lib: None
bq: None
apiclient: None
```
