Merged be.target-as-bug

Highlights:
 * targets are now a special type of bug (severity 'target'), so you
   can do all the things you do with normal bugs to them as well
   (e.g. comment on them, link them into dependency trees, etc.)
 * new command `be due` to get/set bug due dates.
 * changes to `be depend`
   * added options --status, --severity
 * changes to `be list`
   * added blacklist capability to --status, --severity, --assigned
   * removed options --target, --cur-target
   Replace:
     'be list --target TARGET' with
     'be depend --status -closed,fixed,wontfix --severity -target \
        $(be target --resolve TARGET)'
     'be list --cur-target' with
     'be depend --status -closed,fixed,wontfix --severity -target \
        $(be target --resolve)'
 * changes to `be target`
   * added option --resolve
   * removed option --list
   Replace:
     'be target --list' with 'be list --status all --severity target'
 * new function cmdutil.select_values() for whitelist/blacklist selection.
 * assorted cleanups and bugfixes