Added optional support for https

https support is optional. It is enabled if a certificate and a private key are found. Example configuration:

```
{
  "mongo_server": "127.0.0.1",
  "mongo_options": {
    "db": { "native_parser": true}
  },
  "mongo_db": "agora_dev",
  "ssl_options": {
  	"ssl_key": "ssl/server.key",
  	"ssl_cert": "ssl/server.crt",
  	"secureProtocol": "TLSv1_method"
  },
  "nodejs_port": 8081
}
```