*: loads of walking features

For the most part, all the keywords for a standard mtree spec now have a
function to produce the contents for a creator.

These are used in the "walk" function, and will be used next in the
"check" logic.

This is still a WIP, as the DirectoryHierarchy produced from the current
Walk() is not all-together a valid document.

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

walk_test.go

@@ -0,0 +1,16 @@
+package mtree
+
+import (
+	"os"
+	"testing"
+)
+
+func TestWalk(t *testing.T) {
+	dh, err := Walk(".", nil, append(DefaultKeywords, "cksum", "md5", "rmd160digest", "sha1", "sha256", "sha512"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err = dh.WriteTo(os.Stdout); err != nil {
+		t.Error(err)
+	}
+}