Add __getitem__ to Time

This allows one get a slice from a Time array:
```
from astropy.time import Time
t1 = Time(np.arange(2450000,2450010), format='jd', scale='utc')
t1-t1[0]
```
yields
```
<TimeDelta object: scale='tai' format='jd' vals=[ 0.  1.  2.  3.  4.  5.  6.  7.  8.  9.]>
```