New watch API, fixes to export, views, lca computation. Remove Snapshots.

• Fix Irmin.export for the HTTP backend (#196, patch from Alex Zatelepin)
• Fix a race in Irmin.export (#196, patch from Alex Zatelepin)
• Add Task.empty (the empty task) and Task.none (the empty task constructor)
• Completely rewrite the notification mechanism. All the watch functions now
  take a callback as argument and return a de-allocation function. The callbacks
  receive a heads values (the last and current ones) and diff values. (#187)
  • Add Irmin.watch_head to watch for the changes of the current branch's head
  • Add Irmin.watch_tags to watch for the changes of all the tags in the store
  • Add Irmin.watch_key to watch for the changes of the values associated to a
    given key (this is not recursive anymore).
  • Add View.watch_path to watch for the changes in a subtree. The function
    return views and the user can use View.diff to compute differences between
    views if needed.
• Transfer the HTTP client task to the server to make the commit messages
  relative to the client state (and not the server's) (#136)
• Fix View.remove to clean-up empty directories (#190)
• Fix the ordering of tree entries in the Git backend (#190)
• Allow to create a new head from a view and a list of parents with
  View.make_head (#188)
• Allow to create an empty temporary branch with Irmin.empty (#161)
• Use a pure OCaml implementation of SHA1, do not depend on nocrypto anymore
  (#183, by @talex5)
• Remove Irmin.Snapshot. Nobody was using it and it can be easily replaced by
  Irmin.head, Irmin.watch_head and Irmin.update_head.
• Change signature of Irmin.iter to include the values and move it into
  the Irmin.RO signature.
• Add Irmin.fast_forward_head (#172)
• Add Irmin.compare_and_set_head (#171)
• Simplify the RW_MAKER signature (#158)
• Fix Irmin_git.RW_MAKER (#159)
• Improve the efficiency of the LCA computation (#174, with @talex5 help)
• By default, explore the full graph when computing the LCAs. The previous
  behavior was to limit the depth of the exploration to be 256 by default.