Updates Gemfile

Adds koala, twitter, and dotenv-rails.
Adds nokogiri to Gemfile
Adds initializers for Twitter & Facebook APIs

Defines global variables `$client` and `$graph` for accessing the Twitter and Facebook APIs, respectively. Also adds `.env_example` to help show what keys are required for working with the APIs.
Adds ibc:bucket_list rake task

This task scrapes the Wikipedia page listing Ice Bucket Challenge participants and writes their full names to the file `db/bucket_list.txt`.