resample() should allow a "null" fill_method
I'm using `resample()` to aggregate data in a timeframe.

``` python
>>> s = pandas.Series((3, 4), (pandas.Timestamp("2014-1-1"), pandas.Timestamp("2015-1-1")))
>>> x = s.resample("1s")
>>> len(x)
31536001
```

When doing such a call, `resample` fills with NaN all the (31536001 - 2) inexistent values, which ends up creating thousands of points and making Python using 500M+ RAM. The thing is that I don't care about the NaN point, so I would like to _not_ fill them in the `Series` and having so much memory used. AFAICS `resample` does not offer such as `fill_method`.
