Fix regression in the Git backend, add HTTP API versionning

• Add a version check for HTTP client and server. The client might add the
  version in the HTTP headers using the X-IrminVersion header - the server
  might decide to enfore the version check or not. The server always reply
  with its version in the JSON reply, using a version field. The client
  might use that information to bail out nicely instead of failing because
  of some random unmarshalling errors due to API changes (#167)
• Fix a regression in 0.9.5 and 0.9.6 when inserting new child in Git trees.
  This could cause a tree to have duplicate childs having the same names,
  which would confuse the merge functions, make git fsck and git gc
  complain a lot (with good reasons) and do some fency things with git
  index. The regression has been introduced while trying to fix #190 (the fix
  is in #229)