Add date when Finished button is selected.

[jpguz]

No description provided.

[FerPerales]

As far as I know, posts are sorted by creation date, but think about this scenario:

Writer A created a post Jan 1st 2013 and kept is as a draft
Writer B created a post yesterday and marked as finished the same say
Writer A marks marks his blogpost as finished today

Since only creation day is taken into consideration, blogpost from Writer A will have more priority to be reviewed even though was finished a day before the one from writer B

By adding a finished_at attribute, we can use it to sort some accurately