Lots of various golint fixes

Changes some names to match go conventions
Comments all exported methods
Removes dot imports

common/tarsum.go

@@ -25,6 +25,8 @@ type TarSumInfo struct {
 	Digest string
 }
 
+// InvalidTarSumError provides informations about a TarSum that cannot be parsed
+// by ParseTarSum.
 type InvalidTarSumError struct {
 	TarSum string
 }