Twitter credentials hidden, now uses separate file

Uses credentials.json to access Twitter credentials. credentials.json is
a file with one json object that has the following structure:
{"access_token": "your_access_token_here", "consumer_secret":
"your_consumer_secret_here", "consumer_key": "your_consumer_key_here",
"access_token_secret": "your_access_token_secret_here"}.