ENH: support .fillna(method=,limit=Timedelta)
```
In [2]: s.fillna(method='ffill')
Out[2]: 
0    1.0
1    1.0
2    1.0
3    1.0
4    1.0
dtype: float64

In [3]: s.fillna(method='ffill', limit=3)
Out[3]: 
0    1.0
1    1.0
2    1.0
3    1.0
4    NaN
dtype: float64

In [4]: s = Series([1] + [np.nan]*3)

In [5]: s.fillna(method='ffill', limit=2)
Out[5]: 
0    1.0
1    1.0
2    1.0
3    NaN
dtype: float64
```

wish
```
In [7]: s = Series([1] + [np.nan]*3, pd.date_range('20130101',periods=4))

In [8]: s.fillna(method='ffill', limit=pd.Timedelta('2day'))
ValueError: Limit must be an integer
```

