Fix Analyzer warning

Fix the following Analyzer warning:

```
ShareKit/Classes/ShareKit/Core/Helpers/SHKFile.m:257:5: Potential leak of an object stored into 'result'
```

By converting the `UTITypeForPath:` method into a "` Create`" function `CreateUTITypeForPath` as suggested here http://stackoverflow.com/a/8628172/1049134