Better support for random tag name in the Git backend

No need to prefix all the tag names by refs/heads anymore
Diplay some nice graphs when asking the HTTP server to dump the database

This does not scale very well, but we could try to adapt something like http://cs.brown.edu/people/jcmace/d3/graph.html?id=small.json later (the source is somewhere in https://github.com/brownsys/X-Trace/tree/master/src/main/resources/webui/html/interactive)
The recursive stores now seem to work

It's also easier to embed key contents with a special merge semantics.
Merge pull request #59 from samoht/master

Nice graphs