MashupPlatform.wiring.registerStatusCallback fails when called from operators

MashupPlatform.wiring.registerStatusCallback fails when called from operators. E.g.:

```
Uncaught TypeError: Cannot read property 'wiring' of undefinedregisterCallback @ WirecloudAPICommon.js?v=885f55032761c78b9cfe4e6374b080578aab6683:152(anonymous function) @ main.js:64(anonymous function) @ main.js:66
LogManager.js:99 The operator (GitHub Harvester) was loaded.
```
