Vertical bars

I've added a `bar_orientation` (horizontal/vertical) option for the bar charts. 

![screen shot 2014-12-01 at 15 17 16](https://cloud.githubusercontent.com/assets/95854/5246810/2a2864d6-796d-11e4-9b02-089ab67b2a09.png)

It might need some styling (and almost definitely some refactoring) but I'd appreciate a review when you have some time - if anything needs tweaking/changing, please let me know.