Changeset discussion feature

New ChangesetComment model and join table between users and changsets for comments subscriptions
Made commeting changesets possible via "browse" interface
Comments feed for all changsets, particular changeset
Email notifications of new comments on "subscribed to" changesets
Matt,

Some of these are implementation questions, but others appear to be 
fundamental design questions. Please direct the design questions to me.

Find me on IRC.

- Serge