muddle version 2 stamp file support

The latest version of muddle now supports version 2 stamp files. See "muddle help stamp" (and the associated subcommands). There's a tag "stamp-v2-support" if you like that sort of thing.


Version 2 stamp files support all the information stored in the new Repository class we're now using, and also store checkout options (i.e., typically the "shallow checkout" option for git).

There's legacy support for outputting version 1 stamp files (just in case) - this makes a good try at working, but is obviously not guaranteed.

There's somewhat more trusted support for reading version 2 stamp files, although again it may get corner cases wrong.

I've still to finish writing up the actual format (in the doc string for muddled/verstion_stamp.py), but I figured getting the support out for v2 stamps was worth doing, as otherwise we have the potential of losing information.

Other changes:

• "muddle --version" will now report the branch as well, if it's not master
• the odd support for handling branches in bzr by tacking their name on the end of the repository URL has been removed. I don't believe this should affect anyone.
• there are various bugfixes, in the normal manner, and some extra methods on Repository