Enable twitter integration for access announcements

This commit enables the publishing of door access announcements to Twitter, based on the original work by @tomgrek.

This commit differs from the original work by factoring out the credentials for publishing tweets into a separate config file `api.cfg`.

Also, the code wraps the twitter api call in a try-except block to improve robustness, so that exceptions thrown by twython (e.g. caused by loss of Internet connection)  will not crash the door access system.