Interpolating with method='time' incorrect for resolution higher than daily
The following code (pandas 0.8.0b2):

```
import pandas as pd
index = pd.date_range('1/1/2012', periods=4, freq='12D')
ts = pd.Series([0, 12, 24, 36], index)
new_index = index.append(index + pd.DateOffset(days=1)).order()
ts.reindex(new_index).interpolate(method='time')
```

correctly returns:
2012-01-01     0
2012-01-02     1
2012-01-13    12
2012-01-14    13
2012-01-25    24
2012-01-26    25
2012-02-06    36
2012-02-07    36

But when exchanging days for hours:

```
index = pd.date_range('1/1/2012', periods=4, freq='12H')
ts = pd.Series([0, 12, 24, 36], index)
new_index = index.append(index + pd.DateOffset(hours=1)).order()
ts.reindex(new_index).interpolate(method='time')
```

the result is not correct:
2011-01-01 00:00:00     0
2011-01-01 01:00:00    12
2011-01-01 12:00:00    12
2011-01-01 13:00:00    12
2011-01-02 00:00:00    24
2011-01-02 01:00:00    36
2011-01-02 12:00:00    36
2011-01-02 13:00:00    36
