Update to be able to use Interface Builder to customize cell and layout.

Some developers might prefer to do cell customization in IB. This pull request only contains code that expose necessary IBOutlets.

Sample storyboard in the [development/customizeUsingInterfaceBuilderDemo](https://github.com/mystcolor/UICollectionViewWaterfallLayout/tree/development/customizeUsingInterfaceBuilderDemo) branch

![Screen Shot 2013-04-27 at 12 49 23 AM](https://f.cloud.github.com/assets/852375/431759/4e114a96-ae91-11e2-9502-c325cdced804.png)

![Screen Shot 2013-04-27 at 12 50 38 AM](https://f.cloud.github.com/assets/852375/431764/772b3978-ae91-11e2-81d4-5e566c1d640c.png)