API changes and Git backend

• The command-line tool now looks in the environment for the variable
  IRMIN to configure its default backend
• Add a Git backend
• Add Travis CI scripts to the repo
• Use Lwt_bytes and Lwt_unix instead of the custom-made IrminChannel
• Use bin_prot instead of a custom binary protocol
• Major refactoring: Value is now Blob, Revision is now Commit
  and Tag becomes Reference (rational: consistency with Git names)
• Use core_kernel instead of building a custom Identiable.S
• Use dolog instead of a custom log library
• Use mstruct (mutable buffers on top of cstruct) which is now
  released independently