BUG: Period ops with np.timedelta64 cause unexpected result
`Period` + `np.timedelta64` doesn't raise `TypeError(NotImplemented)` (like `offsets` does), and results in erratic internal representation.

```
import pandas as pd
import numpy as np

p = pd.Period('2011-01-01', freq='D')
p = p + np.timedelta64(2, 'h')
# This raises no error

print(p)
# TypeError: don't know how to convert scalar number to int

p.ordinal
#14977 hours
```

`PeriodIndex` + `np.timedelta64` also doesn't raise `TypeError(NotImplemented)`, and are added ignoring `PeriodIndex.freq`.

```
pidx = pd.period_range('2011-01-01', freq='D', periods=5)
pidx = pidx + np.timedelta64(2, 'h')
pidx
# <class 'pandas.tseries.period.PeriodIndex'>
# [2011-01-03, ..., 2011-01-07]
# Length: 5, Freq: D
```
