Bisect plugin updated

[Jeff Licquia]

Robert Collins wrote:
> Rather than bzr_plugin_version can I suggest 'version_info' - which is
> what bzr looks for on an installed plugin already. This is a 5-tuple
> e.g. (0, 92, 0, 'dev', 0).

[...]

> bzr already exposes an api minimum version - a three-tuple (0, 92, 0)
> and a current version 5-tuple. I think that plugins should depend on the
> api version they need.
> 
> E.g. 'I need 1.2 or later' is a needed api of (1, 2, 0). If the
> bzr_minimum_api is greater than this, its not compatible. If the bzr
> version itself is less than this, its not compatible.

Done.  I've written tests, too, which seem to document my understanding 
of the relationships involved.