Implement State Machine

[toastercup]

This Issue will hold our discussions re: what state engine we go with. It looks like there are two options: state_machine and aasm. The former was abandoned 4 years ago, but now has a fork with a decent amount of activity. The latter looks just as active and fleshed out- therefore, I'd like to go through a quick research phase where we suss out the value of each. I'll be doing some research, but please comment if you have any further insight.

[toastercup]

• For aasm vs state_machine comparison (from 2012): http://stackoverflow.com/a/12696383/2363847
• https://github.com/troessner/transitions has a dependency on AR, but looks nifty and is significantly pared down from our other candidates
• https://github.com/geekq/workflow another state engine recommended by some Redditors

[toastercup]

This was completed as part of #341. transitions was the gem of choice.