Add an OCTComment base class for different comment types

`OCTPullRequestComment`, `OCTCommitComment`, and `OCTIssueComment` share some similar properties and AFAIK, are guaranteed to have them in the API. This pulls out some of those properties into an OCTComment base class. This could also be expressed as a protocol which doesn't necessarily have to live in OctoKit, since the API makes no guarantees as to whether these properties will be shared in the future. Thoughts?