This change makes the loading of the 'zk' gem optional and implicit, as well as
the reliance on Curator being available in the JRuby runtime's classpath.
If neither Zookeeper client implementation are available, an exception will be
raised when attmepting to use Zookeeper-based discovery
Fixes#75
This reduces the need for the librdkafka system dependency and results in
statically linking librdkafka.a against the C extension.
There's a few hacks in extconf.rb specifically to deal with librdkafka's build
system, as well as apply checksumming to the downloaded file
Fixes#8