Interpolate NaT
Hello,

`interpolate` doesn't work with `NaT`
see http://stackoverflow.com/questions/33921795/fill-timestamp-nat-with-a-linear-interpolation/33922824#33922824

Here is a trivial example to show the situation:

```
s = pd.Series(pd.date_range('2015-01-01' , '2015-01-30'), name='t')

s[3], s[4], s[5] = pd.NaT, pd.NaT, pd.NaT
s[13], s[14], s[15] = pd.NaT, pd.NaT, pd.NaT
print(s)

0    2015-01-01
1    2015-01-02
2    2015-01-03
3           NaT
4           NaT
5           NaT
6    2015-01-07
7    2015-01-08
8    2015-01-09
9    2015-01-10
10   2015-01-11
11   2015-01-12
12   2015-01-13
13          NaT
14          NaT
15          NaT
16   2015-01-17
17   2015-01-18
18   2015-01-19
19   2015-01-20
20   2015-01-21
21   2015-01-22
22   2015-01-23
23   2015-01-24
24   2015-01-25
25   2015-01-26
26   2015-01-27
27   2015-01-28
28   2015-01-29
29   2015-01-30
Name: t, dtype: datetime64[ns]

print(s.interpolate())
0    2015-01-01
1    2015-01-02
2    2015-01-03
3           NaT
4           NaT
5           NaT
6    2015-01-07
7    2015-01-08
8    2015-01-09
9    2015-01-10
10   2015-01-11
11   2015-01-12
12   2015-01-13
13          NaT
14          NaT
15          NaT
16   2015-01-17
17   2015-01-18
18   2015-01-19
19   2015-01-20
20   2015-01-21
21   2015-01-22
22   2015-01-23
23   2015-01-24
24   2015-01-25
25   2015-01-26
26   2015-01-27
27   2015-01-28
28   2015-01-29
29   2015-01-30
Name: t, dtype: datetime64[ns]

assert s.interpolate().isnull().sum() == 0
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-150-8a59e397a174> in <module>()
----> 1 assert s.interpolate().isnull().sum() == 0

AssertionError:
```

Kind regards
