ENH: Add fill_value option to resample()
see also #3707

Add a `fill_value` option to `resample()` so that it is possible to resample a `TimeSeries` without creating `NaN` values. If the series is an int dtype and an int is passed to `fill_value`, it should be possible to resample the series without casting the values to floats.

```
>>> dates = (datetime(2013, 1, 1), datetime(2013,1,2), datetime(2013,3,1))
>>> s = Series([1,2,4],index=dates)
>>> s
2013-01-01    1
2013-01-02    2
2013-03-01    4
dtype: int64
>>> s.resample('M', how='sum')
2013-01-31     3
2013-02-28   NaN
2013-03-31     4
Freq: M, dtype: float64
>>> s.resample('M', how='sum', fill_value=0)
2013-01-31     3
2013-02-28     0
2013-03-31     4
Freq: M, dtype: int64
```
