New visualiser to display failing tasks and dependency graphs

New user interface intended to replace the current graph view. Task list display shows currently failing tasks and also tasks with failures upstream of them, and the dependency graph will show you exactly why a task is failing. Because it filters to display only items which are upstream of a given task, it should perform a lot better than the existing dependency graph. Error nodes are clickable and will give you a traceback for that task if it is available.

![screen shot 2013-05-27 at 4 36 57 pm](https://f.cloud.github.com/assets/1446837/568490/eb805996-c6da-11e2-8bfc-137f6965d13a.png)

![screen shot 2013-05-27 at 4 31 01 pm](https://f.cloud.github.com/assets/1446837/568473/161f72d2-c6da-11e2-9858-e9e4044fb36b.png)

![screen shot 2013-05-28 at 4 17 37 pm](https://f.cloud.github.com/assets/1446837/572730/86738cae-c7a1-11e2-93ff-3286b5528624.png)