checkbugs update

- milestone checks updated to match current practice of using flags
  to define the 1.0 backlog, with milestones marking incremental
  releases along the way
- now complains if bugs reach ASSIGNED or later without a target
  milestone
- factored out helper classes for each data source
- more comments describing the script's purpose
- initial version of the JIRA consistency checking

Known limitations of JIRA integration:

- when filtering based on release or milestone, complains about
  bugs that were dropped from a sprint after work had been done
- JIRA/Agile based sprint filtering is not yet supported
- Kerberos authentication is not yet supported

Change-Id: I1e4194787d82b0015bc672211973f5efff9cbb7f
Merge "checkbugs update" into develop