started on repos/commits api; added DRY `withOptionalParameter` helper
as we keep repeating the code to build up url params wverywhere
refs #37
implemented repos/.../commits/ list
refs #37
added since / until and moved to "opts" style for repo/commits
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implemented ;get-a-single-commit' in the repository, though it's a bit
duplicated...
The API in git/ does the exactly same thing. - implemented previously by
@willnoris as I just notived

refs #37
Starting to implement repos/compare API - WIP
refs #37