Add Ability to fetch a single commit.
Add Ability to fetch a single commit.

kasperpeulen/github.dart,marcojakob/github.dart,marcojakob/github.dart,kasperpeulen/github.dart,marcojakob/github.dart,kasperpeulen/github.dart,kasperpeulen/github.dart,bwu-dart-contributing/github.dart,marcojakob/github.dart,bwu-dart-contributing/github.dart,bwu-dart-contributing/github.dart,bwu-dart-contributing/github.dart