Merge pull request #24 from vincentbernat/fix/version

version: store version in one place
Merge pull request #25 from vincentbernat/fix/inexistent-file

watcher: handle non-existent configuration file
Merge pull request #26 from vincentbernat/fix/joiner-with-files

join: make join work on files and not just directories
joiner: add an option to work with a common node

In normal operations, each joiner will have its own zookeeper node. We
add an option to make them work on a common node instead. The hostname
is not added to the node and local modifications are not
authoritative. Moreover, the node is not ephemeral.