Add Estimate and Actual time keyins to tickets

[nathanbrizzee]

With Estimate and Actual times on tickets, you are one step closer to Agile tracking. Since milestones have end dates, this program is close but without the ability to estimate how long a task is going to take and the ability to record how long it really took, there's no way to estimate how long a milestone will really take and whether it is on time or overrunning. Adding time tracking to tickets is the first step. The next step would be to add a burn down chart to the milestone view.

[flaix]

This is related to #1014, trying to make Gitblit into a full project planning/tracking tool.