Multiplication/division of TimeDelta

While trying to write a quick script for looking at observability for a range of dates, I noted that something like
```
from astropy.time import Time
import numpy as np
t1 = Time('2013-06-01', scale='utc')
t2 = Time('2013-07-01', scale='utc')
dt = t2-t1
for t in t1+dt*np.linspace(0,1,10):
    print dt
```
did not work. Thinking that there was no specific reason why a `TimeDelta` could not be multiplied with or divided by a number or array, I made the attached patch (which also implements negation). Does this make sense? (note that this alone does not quite resolve the above, as adding a single `Time` to a `DeltaTime` array does not work; see #1081)