Merge assorted bugfixes and optimizations.

Highlights:
  * `be new` adds creator field like its supposed to (oops :p).
  * `be list --xml` uses <be-xml> format (was <bugs>).
  * `be import-xml` handles root comments appropriately.
  * `be` raises an appropriate help message.
  * `be help` works.
  * `be html` prints Comment.id.user() information.
  * better SavedSettingsObject._get_saved_settings() avoids data loss.
  * be-mbox-to-xml -> be-mail-to-xml and adds assorted format support.
  * be-xml-to-mbox and be-handle-mail work with new libbe layout.
  * BugDir.uuids() now caches on-disk uuids for speed.
  * Mercurial detection works for mercurial <= 1.1.2 _and_ >= 1.2
  * Fix bugs in VCS._children() relative/absolute path handling.