Add envvars for deploying to cloud providers

Added some more environment variables to the code. 
It makes easier to deploy heroku. Also, when developing locally, permits you to connect to OAuth providers without modifying production.js/development.js/test.js (or commiting keys/secrets to VCS by mistake).

## Heroku ##
heroku config:add \
  FACEBOOK_ID='facebook-id' \
  FACEBOOK_SECRET='facebook-secret' \
  TWITTER_KEY='twitter-id' \
  TWITTER_SECRET='twitter-secret' \
  GOOGLE_ID='google-id' \
  GOOGLE_SECRET='google-secret' \
  GITHUB_ID='github-id' \
  GITHUB_SECRET='github-secret'\
  SESSION_SECRET='your session secret for express sessions' \
  NODE_ENV='production' \
  APP_URL='your-app-name-in.heroku.com'

## Locally ##
export \
 FACEBOOK_ID='facebook-id' \
  FACEBOOK_SECRET='facebook-secret' \
  TWITTER_KEY='twitter-id' \
  TWITTER_SECRET='twitter-secret' \
  GOOGLE_ID='google-id' \
  GOOGLE_SECRET='google-secret' \
  GITHUB_ID='github-id' \
  GITHUB_SECRET='github-secret'\
  SESSION_SECRET='your session secret for express sessions' \
  PORT=9000 \
  NODE_ENV='development'
  APP_URL='your-app-name-in.heroku.com'

Cheers