hg purge purges files in .ignore

[Marcin Kasperski]

> files autogenerated by the build system can be purged by the
> clean systems clean command, no need for mercurial.

I think both use cases can be satisfied.

	hg purge

should purge only the files displayed by hg status, but there can 
be sth like

       hg purge --all

which would remove also ignored files.