Refactor specification of supported digests

To make the definition of supported digests more clear, we have refactored the
digest package to have a special Algorithm type. This represents the digest's
prefix and we associated various supported hash implementations through
function calls.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

Godeps/_workspace/src/github.com/stevvooe/resumable/sha256/resume.go

@@ -3,6 +3,9 @@ package sha256
 import (
 	"bytes"
 	"encoding/gob"
+
+	// import to ensure that our init function runs after the standard package
+	_ "crypto/sha256"
 )
 
 // Len returns the number of bytes which have been written to the digest.

Godeps/_workspace/src/github.com/stevvooe/resumable/sha512/resume.go

@@ -3,6 +3,9 @@ package sha512
 import (
 	"bytes"
 	"encoding/gob"
+
+	// import to ensure that our init function runs after the standard package
+	_ "crypto/sha512"
 )
 
 // Len returns the number of bytes which have been written to the digest.