Tag activities with runtime

@mattgillooly, please review.
/cc @dstuebe, @joshchu 

Context:
We'd like to be able to visualize the run time for components of a pipeline to more easily visualize bottlenecks etc. This adds run time tracking to each activity.

Example:
![run_time_enhanced_graph](https://cloud.githubusercontent.com/assets/1381881/5489072/c73373c8-8693-11e4-94d9-3bcb3749d950.png)

Do we have any rspec for these?