SplitViewport added

I have seen a few people in the IRC channel struggling with the `Viewport` classes trying to get some split screens working. The ones currently available cannot do anything like this.

This new `SplitViewport` is supposed to help with the problem of split screens with variable screen sizes. It gets initialized with a "root" viewport which determines the area that the sub views can use. Those sub views can be constructed in a pretty simple way by adding rows and cells (viewports) with size informations. It is a bit similar to `Table`. To show how it works I have added a test as well.

A preview with 4 different viewports:
![split](https://cloud.githubusercontent.com/assets/5896510/3869887/d5f80b0e-20ad-11e4-927f-bd0ae2557b46.png)