handle 'Bad credentials' error

• When receiving 'Bad credentials' error message from the API, stop and notify user to re-init.
• Fix wrong version number reported by gister version.

export gist to another git repo

v2.4.0

- Add an `export` command to export gist to an existing git repo.
- `sync` can also clone repository with https protocol (controlled by an
environment variable).
- `sync` can auto commit changes if the gist repo is dirty (controlled
by an environment variable).

v2.3.0: - New command `check` to report dirty repositories.

- `sync` also sync starred gists.
- Use legit to do smart merge if available.
- Less verbose output.
- Fix a bug regarding gist home path expansion.
- Replace Rake with make.

v2.2.0: - sync: skip sync dirty repos, warn instead.

- sync: change UI to report non-modified gist.
- update repo/$gist_id/config for deleted gists
- Fall back to `grep` if codesearch is not available.

v2.1.2

• sync: Compare update time to skip already up to date repos.
• Use gsed and gdate on Mac OS X. Fix #14.

2.0.3-alpha: Fix a bug that causes `gister` to fail if no token.

• redesign UI
• add init function
• replace fetch_list() with fetchall(), which fetches all your gists.
• improve documentation
• add support for Mac OS X and Cygwin

Thanks to wenLiangcan.