Support display of bounded-depth graph (by default, depth=4)

This make it possible to display normal Git repo

to change the default depth, connect to http:/localhost:8080/dump&depth=xx
You can also add 'reload=false' if you don't want the page to auto-reload every seconde.
Add a hackish way to connect to a remote Git repo

Need to integrate that properly at one point
Create an IrminStore.S signature

Would be good to not expose the the IrminBranch.S sig at one point
(but that means that we don't have access to the block and tag stores
anymore).