The command-line tool now looks in the env variables for the default store configuration

Use:
- IRMIN=m        -> in-memory
- IRMIN=g        -> git
- IRMIM=l:<path> -> local
- IRMIN=r:<uri>  -> remote

You can still force the command-line tool selection by adding some options to the command line.
Improve some outputs
`irmin dump` can also choose the store backend
Change the way we represent trees.

Until now, each node could carry a value (a la xenstore).
From now on, only the leaves can carry a value (a la git).

That's easy enough to revert later, and it's easier to map the Git backend.