manifest can be deleted only by digest and not tag

Fixes issue #67.

As per dist spec, DELETE of a image manifest can only be done with
digest as <reference> param. Previously, tags were being allowed as
well. This is not conformant to the spec.

pkg/storage/storage_test.go

@@ -115,6 +115,9 @@ func TestAPIs(t *testing.T) {
 					_, _, _, err = il.GetImageManifest("test", d.String())
 					So(err, ShouldBeNil)
 
+					err = il.DeleteImageManifest("test", "1.0")
+					So(err, ShouldNotBeNil)
+
 					err = il.DeleteImageManifest("test", d.String())
 					So(err, ShouldBeNil)