undefined is not a function

From: https://github.com/shama/gaze/blob/master/lib/gaze.js#L260 it is possible that there would be no callback provided as described in https://github.com/shama/gaze/blob/master/lib/platform.js#L118 . I just added a case for when there is no callback provided. I needed this fix for a project of mine. 

Other wise great work Kyle Robinson Young :)

This fix is for the following error

```
TypeError: undefined is not a function
    at Function.platform.close (/server/node/eve/node_modules/gaze/lib/platform.js:112:14)
    at Gaze._trigger (/server/node/eve/node_modules/gaze/lib/gaze.js:249:14)
    at Object.cb (/server/node/eve/node_modules/gaze/lib/platform.js:49:7)
    at /server/node/eve/node_modules/gaze/lib/statpoll.js:32:20
    at iterate (/server/node/eve/node_modules/gaze/lib/helper.js:106:5)
    at /server/node/eve/node_modules/gaze/lib/helper.js:116:11
    at /server/node/eve/node_modules/gaze/lib/statpoll.js:47:5
    at iterate (/server/node/eve/node_modules/gaze/lib/helper.js:106:5)
    at /server/node/eve/node_modules/gaze/lib/helper.js:116:11
    at /server/node/eve/node_modules/gaze/lib/statpoll.js:47:5
```
Merge pull request #150 from cblanquera/master

undefined is not a function