Implement get_friends_ids() and get_follower_ids()

API for getting list of friends is at
https://dev.twitter.com/docs/api/1.1/get/friends/ids

API for getting list of followers is at
https://dev.twitter.com/docs/api/1.1/get/followers/ids

Properly checks the rate limit status for both API resources. Rate limit is
15 calls every 15 minutes.
Add builder --friends and --followers commands

These builder flags will add lists of followers and friends to each
user's document under the `friend_ids` and `follower_ids` fields.

- Implement _add_followers and _add_friends methods
- Add --friends and --followers to README

Fixes #4 and #3.