Support v1.1 of Twitter API

The new twitter API requires authentication. To use
astrotweet we now require that users sign up their
installation with twitter, then save those credentials
into a JSON file at `$HOME/.astrotweet.json`. The
format is specified in the README.

Fixes #1.

- add authentication notes to README
- add read_credentials function to read credentials from
  `$HOME/.astrotweet.json` file.
- refactored v1.1 twitter connection into connect() function