Updated tags support

[Joachim Nilsson]

Matthew D. Fuller wrote:

>Command-wise, I'd favor filing everything under one `bzr tag` command.
>Proliferation of top-level commands is unpleasant.
>

*AOL*

I've made quite a few mistakes myself having moved and removed
tags from CVS repositories.  So I would even consider leaving out
any way for a regular user to perform such operations. If the wrong
revision is tagged there's always undo.

Regards
 /Jocke