yet more missing attribute readers on goal object

[bsoule]

Per #15 we added some attribute readers, but there are still more that are documented in api.beeminder.com which are missing from the gem.

[kingdonb]

http://kb.commits.to/beeminder-gem-send-pr-for-issue-23

[kingdonb]

Is "panic" still in the API? It does not appear to be documented at http://api.beeminder.com/#goal

[dreeves]

Confirmed that the "panic" field is no longer used. Thanks so much for helping, btw!