Add JSON serialization to Lock File

[skalnik]

We wanna be able to post misc meta data when printing that can be fetched back later. As a start, the name of who kicked off the job would be great. We could also have the print script update the lock file JSON, so the status page could read from that instead of having to parse the logs.

[sshirokov]

I kinda wanna kill this issue and either open separate feature requests issues for the things brought up here, or wait until they become important elsehow, but this isn't really part of the Open Source milestone, I don't think, so I'm moving it out of the milestone.