UX

addresses #9

New flags, note the use of time values on the intervals and jitter:

```
  -agents=100: max number of agents to run concurrently
  -carbon="localhost:2003": address of carbon host
  -flush-interval=10s: how often to flush metrics
  -jitter=10s: max amount of jitter to introduce in between agent launches
  -metrics=10000: number of metrics for each agent to hold
  -prefix="bench": prefix for metrics
  -spawn-interval=10s: how often to gen new agents
```