Start adding analytics

Getting started with adding some analytics, using [keen.io](http://keen.io), as discussed in #239. Pushed through a couple sample events and it was working. Still need:

- [ ] add tests
- [ ] ability to enable or disable through command line
- [ ] ensure that if analytics isn't present in global config, it's created
- [ ] add clear instructions to the readme on how to disable if desired
- [ ] should we add [sentry's global error handler](https://github.com/mattrobenolt/raven-node#catching-global-errors)?
- [ ] make it faster, this slows down commands by ~1s which is no good
- [ ] make sure events are not triggered by tests (environment variables?)