Use the Maybe type when building a new Comment from json for commit_id and path.

I ran into a JsonError when trying to create a comment on an issue since Comment's commit_id is an optional value, so I am marking it and deserialize it as such. Same thing for Comment's path.
The code that was failing was:
```haskell
import Github.Auth
import Github.Issues.Comments
let auth = GithubOAuth "token"
createComment auth "repoowner" "reponame" 34 "Adding comment from ghci"
```
The error message was:
```json
Left (JsonError "key \"commit_id\" not present on the JSON: Object fromList [(\"user\",Object fromList [(\"organizations_url\",String \"https://api.github.com/users/nataren/orgs\"),(\"subscriptions_url\",String \"https://api.github.com/users/nataren/subscriptions\"),(\"followers_url\",String \"https://api.github.com/users/nataren/followers\"),(\"gists_url\",String \"https://api.github.com/users/nataren/gists{/gist_id}\"),(\"gravatar_id\",String \"e5363f7320b1c3f8cb7cb770f9658c40\"),(\"received_events_url\",String \"https://api.github.com/users/nataren/received_events\"),(\"login\",String \"nataren\"),(\"starred_url\",String \"https://api.github.com/users/nataren/starred{/owner}{/repo}\"),(\"id\",Number 40917.0),(\"avatar_url\",String \"https://avatars.githubusercontent.com/u/40917?v=2\"),(\"following_url\",String \"https://api.github.com/users/nataren/following{/other_user}\"),(\"repos_url\",String \"https://api.github.com/users/nataren/repos\"),(\"html_url\",String \"https://github.com/nataren\"),(\"site_admin\",Bool False),(\"url\",String \"https://api.github.com/users/nataren\"),(\"events_url\",String \"https://api.github.com/users/nataren/events{/privacy}\"),(\"type\",String \"User\")]),(\"body\",String \"Adding comment from ghci #1\"),(\"id\",Number 5.1736546e7),(\"updated_at\",String \"2014-08-11T02:23:48Z\"),(\"html_url\",String \"https://github.com/EternalGoldenBraid/TestRepo1/pull/23#issuecomment-51736546\"),(\"url\",String \"https://api.github.com/repos/EternalGoldenBraid/TestRepo1/issues/comments/51736546\"),(\"issue_url\",String \"https://api.github.com/repos/EternalGoldenBraid/TestRepo1/issues/23\"),(\"created_at\",String \"2014-08-11T02:23:48Z\")]")
```
@jwiegley and @maxpow4h Could you review the change?