From the Github entry for twarc:
twarc is a command line tool and Python library for archiving Twitter JSON data. Each tweet is represented as a JSON object that is exactly what was returned from the Twitter API. Tweets are stored as line-oriented JSON. Twarc will handle Twitter API's rate limits for you. In addition to letting you collect tweets Twarc can also help you collect users, trends and hydrate tweet ids.
twarc was developed as part of the Documenting the Now project which was funded by the Mellon Foundation.