Moved Twitter consumer key & secret from source to environment variables

I think it is a safer practice to store the authentication credentials in environment variables rather than hard coded in the source (which may even be stored in a public repository).

Also, I modified the README to have a little more information about setting up the environment once cloned and an alternative way to run the app.

(BTW, if you accept this pull request, I would like to make more contributions as well. I think node on the backend and Angular on the front-end is a clean approach when you want to support multiple UI front-ends such as web, iOS, and Android)