Version bump to v0.5.0

Quite a “large” release with a ton of changes, ranging from a rewrite of
how “configuration attributes” are managed to minor fixups:

* Removal of `zookeeper` recipe. Use a dedicated cookbook for that.
* Host all “configuration attributes” under a `broker` namespace.
* Configurable and sane log4j properties (see `attributes/default.rb`
  for examples).
* Configurable KAFKA_GC_LOG_OPTS environment variable through the
  `gc_log_opts` attribute.
* Bugfix for using this cookbook with AWS OpsWorks (thanks to @pfleidi).