Some corrections to Commits, and added Comments.
Some corrections to Commits, and added Comments.

Modifications:
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- options parameter for the Github constructor is now optional, so you can instantiate with `var github = new Github()` to access public data without specifying authorization information.
- getCommits was failing when only passed a callback. options parameter is now optional.
- corrected path (should not contain '/git/'), removed unnecessary argument `branch` from `getCommit(sha, cb)`, and simplified cb.

Added:
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- Get commit comments: `getComments([sha,] cb)`
- Create a commit comment: `createComment(sha, body, [options,] cb)`
- Update a commit comment: `updateComment(id, body, cb)`

lukeburns/github