to_datetime returns NaT for old dates with coerce=True
When use `coerce=True` in `pandas.to_datetime` old dates (before 1677-09-22) return NaT. The same dates without `coerce` are parsed correctly:

```
print pd.to_datetime('13000101', format='%Y%m%d', coerce=True)
print pd.to_datetime('13000101', format='%Y%m%d')
```

returns 

```
NaT
1300-01-01 00:00:00
```

pandas: 0.15.2
numpy: 1.9.1
