Add comment option for createBadgeInstance

This adds an optional "comment" that can be specified when calling createBadgeInstance.  Badgekit API does not store this value, but simply passes it along to any webhooks called when creating a badge.

This changes the signature of createBadgeInstance from (context, code, callback) to (context, options, callback), where options can be of the form { code: code, comment: comment}.  The old format of (context, code, callback) is still supported, however.