add ability to retrieve list of topics from nsqlookupd

[stuart-warren]

fairly self explanatory

fixes #37

$ irb
irb(main):001:0> require 'krakow'
D, [2015-07-10T16:59:27.201807 #22698] DEBUG -- : Celluloid 0.17.0 is running in BACKPORTED mode. [ http://git.io/vJf3J ]
=> true
irb(main):002:0> d = Krakow::Discovery.new(:nsqlookupd => ["http://server1:4161"])
=> <Krakow::Discovery:70146309073700 [{:nsqlookupd=>["http://server1:4161"]}]>
irb(main):003:0> d.topics
D, [2015-07-10T16:59:50.116443 #22698] DEBUG -- : <Krakow::Discovery:70146309073700>: Requesting lookup of topics - http://server1:4161/topics
D, [2015-07-10T16:59:50.128586 #22698] DEBUG -- : <Krakow::Discovery:70146309073700>: Lookup response (http://server1:4161/topics): {"status_code"=>200, "status_txt"=>"OK", "data"=>{"topics"=>["logs.system.docker", "logs.system.marathon", "logs.docker.test1"]}}
D, [2015-07-10T16:59:50.128668 #22698] DEBUG -- : <Krakow::Discovery:70146309073700>: Discovery lookup result: ["logs.system.docker", "logs.system.marathon", "logs.docker.test1"]
=> ["logs.system.docker", "logs.system.marathon", "logs.docker.test1"]
irb(main):004:0>
D, [2015-07-10T17:00:28.188155 #22698] DEBUG -- : Terminating 5 actors...