Bugfix: Array * TimeDelta fails for mysterious reason

Multiplying an `nd.array` with a `TimeDelta` fails, as the numpy function tries to get an item that does not exist (independent of whether `TimeDelta` is an array or not. It seems `TimeDelta` never gets a chance to try (the reverse -- `TimeDelta * Array` works fine); somehow, numpy fails to return `NotImplemented`? Mysterious.
```
In [1]: from astropy.time import Time, TimeDelta; import numpy as np

In [2]: np.array(1)*TimeDelta(100., format='sec')
ERROR: TypeError: scalar 'TimeDelta' object is not subscriptable. [astropy.time.core]
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-91a8a4ea9a5c> in <module>()
----> 1 np.array(1)*TimeDelta(100., format='sec')

/home/mhvk/packages/astropy/astropy/time/core.py in __getitem__(self, item)
    506         if self.isscalar:
    507             raise TypeError('scalar {0!r} object is not subscriptable.'.format(
--> 508                 self.__class__.__name__))
    509         tm = self.replicate()
    510         jd1 = self._time.jd1[item]

TypeError: scalar 'TimeDelta' object is not subscriptable.
```