• Fix the invalidation of the view caches (report by @gregtatcam).
  This was causing some confusing issues where views' sub-keys where
  not properly updated to to their new values when the view is merged
  back to the store. The issues is a regression introduced in 0.9.0.
• Add post-commit hooks for the HTTP server.
• Add Irmin.watch_tags to monitor tag creation and desctructions.
• Fix Irmin.push
• Add Irmin.with_hrw_view to easily use transactions.
• Add a phantom type to Irmin.t to denote the store capabilities
  read-only, read-write or branch-consistent.
• The ~old argument of a merge function can now be optional to
  signify that there is no common ancestor.
• Expose Irmin.with_rw_view to create a temporary, in-memory and
  mutable view of the store. This can be used to perform atomic
  operations in the store (ie. non-persistent transactions).
• Simplify the view API again
• Expose the task of previous commits. This let the user access
  the Git timestamp and other info such as the committer name (#90)
• The user-defined merge functions now takes an unit -> 'a result Lwt.t argument for ~old (instead of 'a). Evalutating the
  function will compute the least-common ancestors. Merge functions
  which ignore the old argument don't have to pay the cost of
  computing the lcas anymore.
• Expose S.lcas to get the least common ancestors
• Update to ocaml-git 1.4.6