Build/Test harness with dockerfile and makefile

Signed-off-by: Erik Hollensbe <erik@docker.com>
Signed-off-by: Solomon Hykes <solomon@docker.com>
remove vendor dir now that this is no longer necessary
Keep TestEmpty from panicking
tag error messages in pipeline operations.
fix AssertEq: resolves a lot of tests. :)
Pass the repository to the Pipeline; resolves all panics in tests.
Commit on empty refspec for (*Repository).DB(ref string)
Remove dead code in (*Tree).Set