Logstash plugin introduced

 - logs can now be redirected to the logstash instance

Typical config:
```
...
"loggers": {
  "core": {
    "type": "logstash",
    "args" : {
      "host": "0.0.0.0",
      "port": 50030,
      "verbosity": "debug"
    }
  }
}
...
```
Merge branch 'master' of github.com:cocaine/cocaine-plugins

Conflicts:
	CMakeLists.txt