use 'normal closure' close code by default, fixes #127

SocketRocket uses the invalid close code -1 as the value in the default close method. In addition to being invalid, it causes issues when communicating with some libraries. See section 11.7 WebSocket Close Code Number Registry in the WebSocket Protocol RFC 6455 for more detail.
use normal close constant instead of hard-coded value
Merge pull request #157 from pizthewiz/valid-close-code

use 'normal closure' close code by default, fixes #127