Close Graylog2 connection on close. Fixes #24

Fix an issue where the Graylog2 transport ignores requests
from Winston to close the transport. This causes applications
depending on Winston's close() functionality to hang.

Caveat: currently there is no way to pass Graylog2's close()
callback to Winston, since Winston does not support this.

lib/winston-graylog2.js

@@ -97,4 +97,8 @@ Graylog2.prototype.log = function(level, msg, meta, callback) {
   callback(null, true);
 };
 
+Graylog2.prototype.close = function() {
+  this.graylog2.close();
+};
+
 module.exports = Graylog2;