Add ldp endpoints for commits

[dlamoris]

/orgs/{orgid}/repos/{repoid}/commits and /orgs/{orgid}/repos/{repoid}/commits/{commitid}

GET, HEAD, PATCH

Since commits are created by doing sparql updates or model loads, there wouldn't be post or put support. Having patch would allow a commit to have more metadata like comments or user added arbitrary triples to elaborate the commit.

Expands on #117