Added support for JSON Github requests

Github can send hook events either in json format or in urlencoded format. Ghprb supports only the latter one, which is confusing and not convenient sometimes. This change adds JSON format support.
I mean 
```
if ("application/json".contentEquals(type)) {
    ...
} else if ("application/x-www-form-urlencoded".contentEquals(type)) {
   ...
} else {
  logger.log(Level.ERROR, "Wrong content type: {0}", type);
}
```