Redesign the API for updates

* The get-modify-update round-trip was inconvenient and difficult
  to get right (stashing was required to avoid overwriting your own data
  with wrong values in some cases)
* The new Update message takes a function that is applied on the the
  current data in the Replicator
* read-repair happens before GetSuccess
* enable pruning of removed node again
Merge pull request #26 from patriknw/wip-rework-update-api-patriknw

Redesign the API for updates
Use test scope for akka-multi-node-testkit dependency
Merge pull request #27 from patriknw/wip-24-multi-node-testkit-patriknw

Use test scope for akka-multi-node-testkit dependency
Release v0.4
Bump version to 0.5-SNAPSHOT
Update DataBot sample in README