• Spring clean-ups in the API. Separation in IrminBranch for
  fork/join operations, IrminSnapshot for snapshot/revert
  operations and IrminDump for import/export operations.
  The later two implementation can be derived automaticaly
  from a base IrminBranch implementation. The update and merge
  operations are supported on each backend
• IrminGit does not depend on unix anymore and can thus be
  compile to javascript or xen with mirage
• No need to have bin_io converter for contents anymore
• No need to have JSON converter for contents anymore
• No more IrminDispatch
• Add an optional branch argument to Irmin.create to use
  an already existing branch
• Fix order of arguments in Irmin.merge