conf: ensure we can encode unicode to PHP

Also, add a test that we are able to decode an UTF-8 encoded JSON
file.
Merge pull request #32 from vincentbernat/fix/unicode-php

conf: ensure we can encode unicode to PHP
conf: don't use keyword `errors` for `str.encode`

This is not supported with Python 2.6. Just use it as a positional
argument.
Merge pull request #35 from vincentbernat/fix/php-unicode

conf: don't use keyword `errors` for `str.encode`
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. Instead of using
an IP address, `common` is used as the node name.
Merge pull request #27 from vincentbernat/feature/join-common-node

joiner: add an option to work with a common node
joiner: split `ZkFarmJoiner` into `ZkFarmImporter` and `ZkFarmJoiner`

 - `ZkFarmExporter` will take configuration from zookeeper and put it
   into the local filesystem.

 - `ZkFarmImporter` will take configuration from the local filesystem
   and put it into zookeeper.

 - `ZkFarmJoiner` will do both.

The new class `ZkFarmImporter` will be used in the future to add the
ability to import conf to zookeeper without pushing the changes to the
local filesystem. To avoid code duplication, it retains the same FSM
than `ZkFarmJoiner` but the `znode modified` state is unreachable
because we do not listen to the appropriate zookeeper node.
README: add missing configuration about `zkfarmer --join --common`
importer: add unittests for new `ZkFarmImporter`

Those tests are shared with tests for `ZkFarmJoiner` when this is
possible.
zkfarmer: add `zkfarmer import` command

This commands will import local configuration to ZooKeeper but will
not update the local configuration with ZooKeeper content. This is not
the exact reverse of `zkfarmer export` since it is only able to import
one node. This is a limited version of `zkfarmer join` where changes
from ZooKeeper are ignored.

This is useful when you have a large farm of joinder using a common
node to sync configuration.
Merge pull request #36 from vincentbernat/feature/zkimporter

zkfarmer import