Series astype() doesn't convert int64 to datetime64 using M8[D] anymore
add `units='D'` to `Timestamp/to_datetime` epoch support

I have an int64 series representing days since epoch which I'd like to convert to datetime64.
This works in 11.0 but raises an exception in 0.11.1.dev-964516a. The exception message is "TypeError: cannot convert datetimelike to dtype [datetime64[D]]".

```
import pandas as pd
s = pd.Series([0,1,2])
print s.astype('M8[D]')
```

My workaround is:

```
import pandas as pd
s = pd.Series([0,1,2])
print pd.Series(s.values.astype('M8[D]'))
```
