Make Sensu::Socket support check results too large to fit in a single packet.

ATTN @portertech 
FYI @grepory
When testing the code, I've found MultiJson seems to mistakenly manage to parse partial JSON.  When it does this, check ends up being parse as a symbol, which is very weird.  

This is the error you get when this happens:

```
/opt/sensu/embedded/lib/ruby/gems/2.0.0/gems/sensu-0.13.1/lib/sensu/socket.rb:129:in `[]': no implicit conversion of Symbol into Integer (TypeError)
	from /opt/sensu/embedded/lib/ruby/gems/2.0.0/gems/sensu-0.13.1/lib/sensu/socket.rb:129:in `process_json'
	from /opt/sensu/embedded/lib/ruby/gems/2.0.0/gems/sensu-0.13.1/lib/sensu/socket.rb:116:in `process_data'
	from /opt/sensu/embedded/lib/ruby/gems/2.0.0/gems/sensu-0.13.1/lib/sensu/socket.rb:88:in `receive_data'
	from /opt/sensu/embedded/lib/ruby/gems/2.0.0/gems/sensu-em-2.4.0/lib/eventmachine.rb:187:in `run_machine'
	from /opt/sensu/embedded/lib/ruby/gems/2.0.0/gems/sensu-em-2.4.0/lib/eventmachine.rb:187:in `run'
	from /opt/sensu/embedded/lib/ruby/gems/2.0.0/gems/sensu-0.13.1/lib/sensu/client.rb:12:in `run'
	from /opt/sensu/embedded/lib/ruby/gems/2.0.0/gems/sensu-0.13.1/bin/sensu-client:10:in `<top (required)>'
	from /opt/sensu/bin/sensu-client:23:in `load'
	from /opt/sensu/bin/sensu-client:23:in `<main>'
```

Line 129 is: 

``` ruby
      check[:status] ||= 0
```

Adding the following before that line shows what MultiJson is doing: 

``` ruby
      @logger.warn('check class', {
        :check_class => check.class
      })
```

Here's the output from the log event:

``` json
{"timestamp":"2014-08-16T16:39:20.451141+0100","level":"warn","message":"check class","check_class":"Symbol"}
```