Specify hipchat server address

Add an ability to connect to self managed HipChat Server.

Yes. It raises an error if I set any value other than `chat.btf.hipchat.com`

```
/home/shu/workspaces/tmp/lita4/.bundle/ruby/2.0.0/bundler/gems/lita-hipchat-9645191c81bc/lib/lita/adapters/hipchat/connector.rb:131:in `load_roster': undefined method `attributes' for nil:NilClass (NoMethodError)
        from /home/shu/workspaces/tmp/lita4/.bundle/ruby/2.0.0/bundler/gems/lita-hipchat-9645191c81bc/lib/lita/adapters/hipchat/connector.rb:32:in `connect'
        from /home/shu/workspaces/tmp/lita4/.bundle/ruby/2.0.0/bundler/gems/lita-hipchat-9645191c81bc/lib/lita/adapters/hipchat.rb:42:in `run'
        from /home/shu/workspaces/tmp/lita4/.bundle/ruby/2.0.0/gems/lita-4.0.1/lib/lita/robot.rb:69:in `run'
        from /home/shu/workspaces/tmp/lita4/.bundle/ruby/2.0.0/gems/lita-4.0.1/lib/lita.rb:75:in `run'
        from /home/shu/workspaces/tmp/lita4/.bundle/ruby/2.0.0/gems/lita-4.0.1/lib/lita/cli.rb:77:in `start'
        from /home/shu/workspaces/tmp/lita4/.bundle/ruby/2.0.0/gems/thor-0.19.1/lib/thor/command.rb:27:in `run'
        from /home/shu/workspaces/tmp/lita4/.bundle/ruby/2.0.0/gems/thor-0.19.1/lib/thor/invocation.rb:126:in `invoke_command'
        from /home/shu/workspaces/tmp/lita4/.bundle/ruby/2.0.0/gems/thor-0.19.1/lib/thor.rb:359:in `dispatch'
        from /home/shu/workspaces/tmp/lita4/.bundle/ruby/2.0.0/gems/thor-0.19.1/lib/thor/base.rb:440:in `start'
        from /home/shu/workspaces/tmp/lita4/.bundle/ruby/2.0.0/gems/lita-4.0.1/bin/lita:6:in `<top (required)>'
        from /home/shu/workspaces/tmp/lita4/.bundle/bin/lita:16:in `load'
        from /home/shu/workspaces/tmp/lita4/.bundle/bin/lita:16:in `<main>'
```