BUG: resample closed='left' not binning correctly.
related: http://stackoverflow.com/questions/21329425/resampling-a-pandas-dataframe-with-loffset-introduces-an-additional-offset-of-an

Hey pandas team. Sorry to have gone MIA the past week, super busy with work. I promise (and look forward to) contributing more soon. :)

Still, I wanted to note that I came across what I believe to a be a bug in `resample()` when trying to change the interval of the binning with `closed='left'`. I know that there have been a few changes to the `resample()` API since Wes' book, however, I don't believe they changed this functionality, but I have been wrong before :)

Bug can be reproduced using the example from Wes' book, generating 12 mins of data like:

``` python
In [3]: rng = pd.date_range('1/1/2000', periods=12, freq='T')
In [4]: ts = pd.Series(np.arange(12), index=rng)
In [5]: ts
Out[5]: 
2000-01-01 00:00:00     0
2000-01-01 00:01:00     1
2000-01-01 00:02:00     2
2000-01-01 00:03:00     3
2000-01-01 00:04:00     4
2000-01-01 00:05:00     5
2000-01-01 00:06:00     6
2000-01-01 00:07:00     7
2000-01-01 00:08:00     8
2000-01-01 00:09:00     9
2000-01-01 00:10:00    10
2000-01-01 00:11:00    11
Freq: T, dtype: int64
```

we can do a simple resample to 5 mins like:

``` python
In [6]: ts.resample('5min', how='sum')
Out[6]: 
2000-01-01 00:00:00    10
2000-01-01 00:05:00    35
2000-01-01 00:10:00    21
Freq: 5T, dtype: int64
```

For my use, I need this resampling to be 'backwards looking' so that the summations at each resampled timestamp include the previous 4 minutes. Documentation (and Wes' book) suggest this is achieved by binning with `closed='left'`, however, this results in the same output as above:

``` python
In [7]: ts.resample('5min', how='sum', closed='left')
Out[8]: 
2000-01-01 00:00:00    10
2000-01-01 00:05:00    35
2000-01-01 00:10:00    21
Freq: 5T, dtype: int64
```

I was looking for the following result (note that the first timestamp is at `00:05:00` and with hanging data dropped):

``` python
2000-01-01 00:05:00    10
2000-01-01 00:10:00    35
Freq: 5T, dtype: int64
```

I am able to generate this by combining `loffset='5min'` and then slicing into the resultant Series to remove the:

``` python
In [10]: ts.resample('5min', how='sum', closed='left', loffset='5min')[:-1]
Out[10]: 
2000-01-01 00:05:00    10
2000-01-01 00:10:00    35
Freq: 5T, dtype: int64
```

but this is hardly ideal as it's not known in advance if time series ends with a timestamp that resolves equally to the final timestamp of the resampling procedure!

Apologies if I am missing something—any thoughts, help or guidance is welcomed! 
Thanks so much.
