Coho updates

[Andrew Grieve]

Spent some time on it yesterday & today. Highlights:
- Much faster start-up on node 0.10 (made some requires lazy)
- Much faster repo-clone and repo-update commands (made network work
concurrent)
- Clearer output for repo-status command (prints nothing for no changes,
prints unpushed changes & uncommitted files)
- Added a "verify-tags" command, so you can paste in Steve's plugins
release vote email and it will check that all the tags match the hashes.