ENH: GH3371 support timedelta fillna
closes #3371
- ENH: GH3371 support timedelta fillna
- BUG: add support for datetime64 ffill/bfill
- INT: add TimeDeltaBlock support in internals

```
In [3]: s = Series([Timestamp('20130101'),Timestamp('20130101'),
                  Timestamp('20130102'),Timestamp('20130103 9:01:01')])

In [4]: td = s.diff()

In [5]: td
Out[5]: 
0                NaT
1           00:00:00
2   1 days, 00:00:00
3   1 days, 09:01:01
dtype: timedelta64[ns]

In [6]: td.fillna(0)
Out[6]: 
0           00:00:00
1           00:00:00
2   1 days, 00:00:00
3   1 days, 09:01:01
dtype: timedelta64[ns]

In [7]: from datetime import timedelta

In [8]: td.fillna(timedelta(days=3,seconds=5))
Out[8]: 
0   3 days, 00:00:05
1           00:00:00
2   1 days, 00:00:00
3   1 days, 09:01:01
dtype: timedelta64[ns]
```

ffill

```
In [9]: td[2] = np.nan

In [10]: td
Out[10]: 
0                NaT
1           00:00:00
2                NaT
3   1 days, 09:01:01
dtype: timedelta64[ns]

In [11]: td.ffill()
Out[11]: 
0                NaT
1           00:00:00
2           00:00:00
3   1 days, 09:01:01
dtype: timedelta64[ns]
```
