BUG: timedelta64 dtype inference in Series
related #5690

This works:

```
pd.Series(np.array([np.timedelta64(300000000)]))
```

```
0   00:00:00.300000
dtype: timedelta64
```

but this does not:

```
pd.Series([np.timedelta64(300000000)])
```

on Numpy 1.7:

```
TypeError: don't know how to convert scalar number to float
```

and on Numpy 1.8:

```
0    300000000
dtype: int64
```
