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 () { /* ... */ }
});
```
Why is `typeof potential.__deref === 'function'` being tested? I don't see reference to `__deref` in immutable-js v3 or v2.

We may improve checks with suggestion from here: https://github.com/facebook/immutable-js/issues/231#issuecomment-66509771