Add an (almost) working dispatcher.

The code is quite ugly at the moment, and the tests are broken, so I suppose this needs more work.
Add missing test
Fix the tests for dispatch

Not sure it really works as expected, but at least it pass the tests!
Speed-up the value multiplexer (IrminValue.Mux)
Remove dead-code
Use a map to store the node successors (instead of an assoc list).
Speed-up the memory backend

Do not serialize the contents to bigstrings, just keep it in the heap.
Do not create a graph on export
Use ocaml-sha instead of cryptokit
Add a bulk update API

This does not create intermediate commit messages.

Note: this could also be used to speed-up things later by computing the object sha1 only when commiting (and not on each change).
Update deps in README
Update deps in .travis-ci.sh
Update autogen files
Debug messages tweaks
Speed-up export
Speed-up insertion
Add an other dispatch backend
Refactor the tests to speed-up compilation
Use new map API for nodes, test bulk updates
More broken tests