Improve formatting for Time and TimeDelta when used in Table

[astrofrog]

When used in tables, the formatting for Time and TimeDelta isn't always clear when not using a well known format such as the ISO formatting. For example in the following it's not clear what the units are:

In [22]: from astropy.table import Table
    ...: from astropy.time import TimeDelta
    ...: from astropy import units as u
    ...: t = Table()
    ...: t['time'] = TimeDelta([1, 2, 3] * u.s)

In [23]: t
Out[23]: 
<Table length=3>
         time         
        object        
----------------------
1.1574074074074073e-05
2.3148148148148147e-05
 3.472222222222222e-05

The 'object' isn't very useful (should this show the actual mix-in class type, such as TimeDelta?) and the units aren't clear. I'm not sure what the best way is to make this more intuitive, but at the moment I feel like the output above is a bit obscure.