Improve View.merge_path + dump graphs improvement

• Support backend specific protocols for push/pull
• The Irmin Git backend can now sync with remote Git repositories
• Simplify the organisation of the libraries: irmin, irmin.backend,
  irmin.server and irmin.unix (check how the example are compiled)
• Small refactoring to ease the use of the API. Now use open Irmin_unix
  at the top of your file and use less functor in your code (again,
  check the examples)