Feature/zendesk

Sample zendesk input plugin.  Initial PR for code review by LS team.
Most (all?) other plugins call this "user":

```
logstash/outputs/rabbitmq.rb:31:  config :user, :validate => :string, :default => "guest"
logstash/outputs/xmpp.rb:14:  config :user, :validate => :string, :required => :true
logstash/outputs/xmpp.rb:20:  config :users, :validate => :array
logstash/outputs/elasticsearch_http.rb:61:  config :user, :validate => :string, :default => nil
logstash/outputs/elasticsearch_river.rb:55:  config :user, :validate => :string, :default => "guest"
logstash/outputs/irc.rb:23:  config :user, :validate => :string, :default => "logstash"
logstash/inputs/rabbitmq.rb:32:  config :user, :validate => :string, :default => "guest"
logstash/inputs/xmpp.rb:18:  config :user, :validate => :string, :required => :true
logstash/inputs/imap.rb:21:  config :user, :validate => :string, :required => true
logstash/inputs/irc.rb:28:  config :user, :validate => :string, :default => "logstash"
```