Separates shouldComponentUpdate to its own file. Making it accessible externally

Have `shouldComponentUpdate` in it's own file, making it accessible by doing:

```js
var shouldComponentUpdate = require('omniscient/updateMixin');

var ShouldComponentUpdate = [{
   shouldComponentUpdate: shouldComponentUpdate
}];

var Component = React.createClass({
   mixins: ShouldComponentUpdate,
   render: function () { /* ... */ }
});
```
This was in the "early" days, and as Immutable changed it's Cursor API, which was later reverted. I've removed this now. Thanks for commenting!