Time fromepoch scale conversion (closes #1122); TimePlotDate correction

The following problem from #1122 now is solved:
```
from astropy.time import Time
t = Time(Time.now().iso, scale='ut1')
t.delta_ut1_utc = 0.0
t.unix
```
no longer yields an `IndexError` (as the `delta_ut1_utc` was lost).

In the process, also a bug in `TimePlotDate` was solved. Now, the epoch scale is `utc`, per the `matplotlib.pylab.date2num` documentation.