log: Make it possible to manipulate the writer

Add functions to make it possible to manipulate the internal log writer.
This can be used to disable logging and stuff.

Change-Id: d2885f2fd3
Story-Id: SF-88
changes: Add StoryChanges

Pull some functionality out of release stage and put it into
StoryChanges. This function can be used to get StoryChangeGroups
associated with the given stories. Include/exclude source filter can be
supplied to filter the changes returned.

Change-Id: 3549f3a657
Story-Id: SF-88
release changes: Bring the command up to date

Change-Id: 998457b48b
Story-Id: SF-88