Show marked-up estimated or true finish time in text view.

Close #1121.

Backward compatibility with 5.4.14 suite daemon tested (shows full datetime string for true T-finish, omits estimate).

@benfitzpatrick - please review.

![gcylc-tfinish](https://cloud.githubusercontent.com/assets/2362137/4086344/6079caa4-2f33-11e4-9b9a-92bf12cc91df.png)


Ah yes, you're right - that was silly of me. I was thinking that the right way to do this is to have the daemon report time info in raw form, and the GUI transform it into whatever display form it needs.  But that would require a bit of extra work in the GUI to avoid recomputing the display string at every update ... so I might as well just move the string calculation back to the daemon.