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)


This is nicer code, but I worry that it'll be slow because of the date-string recalculations. I'll give it a test and see what happens.