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.