• Fix the behavior of IrminMemory.Make to return an hanlder to a
  shared datastore instead of creating a fresh one. Add
  IrminMemory.Fresh to return a fresh in-memory datastore maker.
• The HTTP server now outputs some nice graph (using dagre-d3). Don't
  expect to display very large graphs
• More friendly tag names in the Git backend (no need to prefix
  everything by refs/heads/ anymore)
• Partial support for recursive stores (WIP)