Handle WebSocket connection closing

I found an issue with WebSocket connection closing when client is connected and server tries to close the stream.

```ruby
#!/usr/bin/env ruby
$:<< '../lib' << 'lib'

require 'goliath'
require 'goliath/websocket'

class Websocket < Goliath::WebSocket
  def on_open(env)
    env.logger.info("WS OPEN")
    env['subscription'] = env.channel.subscribe { |m| env.stream_send(m) }
  end

  def on_message(env, msg)
    env.logger.info("WS MESSAGE: #{msg}")
    env.channel << msg
    env.stream_close
  end

  def on_close(env)
    env.logger.info("WS CLOSED")
    env.channel.unsubscribe(env['subscription'])
  end

  def on_error(env, error)
    env.logger.error error
  end
end
```

Then start Goliath, connect, and send message from any WebSocket client. The following output would be produced:

```
eveel@tazik{~/Work/goliath/examples}% ./websocket-closing.rb -s 
[24165:INFO] 2013-01-14 05:46:06 :: Starting server on 0.0.0.0:9000 in development mode. Watch out for stones.
[24165:INFO] 2013-01-14 05:46:11 :: WS OPEN
[24165:INFO] 2013-01-14 05:46:12 :: WS MESSAGE: Rock it with HTML5 WebSocket
[24165:ERROR] 2013-01-14 05:46:12 :: undefined method `close_connection' for #<#<Class:0x000000020bbb00>:0x000000020bba10>
/usr/local/rvm/gems/ruby-1.9.3-p362@goliath/gems/em-websocket-0.3.8/lib/em-websocket/message_processor_06.rb:25:in `message'
/usr/local/rvm/gems/ruby-1.9.3-p362@goliath/gems/em-websocket-0.3.8/lib/em-websocket/framing07.rb:106:in `process_data'
/usr/local/rvm/gems/ruby-1.9.3-p362@goliath/gems/em-websocket-0.3.8/lib/em-websocket/handler.rb:28:in `receive_data'
/home/eveel/Work/goliath/lib/goliath/websocket.rb:17:in `on_body'
/home/eveel/Work/goliath/lib/goliath/request.rb:121:in `call'
/home/eveel/Work/goliath/lib/goliath/request.rb:121:in `parse'
/home/eveel/Work/goliath/lib/goliath/connection.rb:70:in `receive_data'
/usr/local/rvm/gems/ruby-1.9.3-p362@goliath/gems/eventmachine-1.0.0/lib/eventmachine.rb:187:in `run_machine'
/usr/local/rvm/gems/ruby-1.9.3-p362@goliath/gems/eventmachine-1.0.0/lib/eventmachine.rb:187:in `run'
/usr/local/rvm/gems/ruby-1.9.3-p362@goliath/gems/em-synchrony-1.0.2/lib/em-synchrony.rb:28:in `synchrony'
/home/eveel/Work/goliath/lib/goliath/server.rb:73:in `start'
/home/eveel/Work/goliath/lib/goliath/runner.rb:297:in `run_server'
/home/eveel/Work/goliath/lib/goliath/runner.rb:221:in `run'
/home/eveel/Work/goliath/lib/goliath/application.rb:120:in `run!'
/home/eveel/Work/goliath/lib/goliath/application.rb:142:in `block in <module:Goliath>'
[24165:INFO] 2013-01-14 05:46:12 :: Status: 500, Content-Length: 85, Response Time: 861.04ms
[24165:INFO] 2013-01-14 05:46:12 :: WS CLOSED
```

I'm unable to set up local development environment for Goliath, but this issue can be easily resolved by adding the `close_connection` method into the connection handler class.