Revise setup instructions in README.md

* Set up local environment and clarified instructions in `README.md`
* Centralize on localhost:3000, since that's likely the default port
* Clearer GitHub and Stripe instructions

https://trello.com/c/TpFwhtlt/333-replicate-setup-locally-and-update-readme-me
Can we add a step for setting up a plan named "private" on Stripe? https://dashboard.stripe.com/test/plans

`RepoSubscriber#subscribe` ([source](https://github.com/thoughtbot/hound/blob/master/app/services/repo_subscriber.rb#L12-L34)) will raise if there's no plan.

> Create a Stripe plan for your development environment:
    ID: "private"
    Name: "private"
    Amount: $12.00
    Recurring: Monthly

![screen shot 2014-10-15 at 9 29 41 am](https://cloud.githubusercontent.com/assets/1855523/4649250/d8f0bc66-5488-11e4-9755-12e361b97656.png)