Use dotenv-rails instead of straight dotenv

dotenv-rails uses .env.#{Rails.env} if available, which allows us to
configure environment-specific variables. As such we now include the
base .env file with the repo, and instead ignore local development
configuration.
Switch to ENV variable style .env file
Configure Supermarket with ENV variables

The various components that were once configured with magiconf but which
are now configured with ENV variables:

  - StatsD
  - The CLA Location
  - NewRelic
  - SMTP
  - PubSubHubbub secrets
  - OmniAuth
  - Octokit
  - Raven
  - Mailer URL
  - CLA notification recipient
  - Default sender address in ActionMailer
  - the app's secret key base
  - Segment.IO key
  - Paperclip S3 settings
  - the ICLA/CCLA version
  - the PubSubHubbub callback URL
  - Curry's success label
Remove StatsD

We never ended up tracking any metrics with it. If/when we do, this
commit should revert cleanly (Gemfile and Gemfile.lock notwithstanding)
Remove non-essential variables from .env

SMTP, Sentry, S3, and New Relic are all handy to tune in production, but
just add noise in development and test. For Sentry and SMTP, fake
settings can cause unusual errors, as well.

Finally, CHEF_OAUTH2_SITE is unused, so we just remove it.
Expand CONFIGURING.md with all variables and dotenv info