Remove non-relative Seek calls from all functions.

Previously we would seek to the beginning of the io.ReadSeeker. It is much more
flexible to assume that the caller passes the ReadSeeker in an appropriate state.

tag.go

@@ -7,7 +7,9 @@ package tag
 
 import (
 	"errors"
+	"fmt"
 	"io"
+	"os"
 )
 
 // ErrNoTagsFound is the error returned by ReadFrom when the metadata format
@@ -25,6 +27,11 @@ func ReadFrom(r io.ReadSeeker) (Metadata, error) {
 		return nil, err
 	}
 
+	_, err = r.Seek(-11, os.SEEK_CUR)
+	if err != nil {
+		return nil, fmt.Errorf("could not seek back to original position: %v", err)
+	}
+
 	switch {
 	case string(b[0:4]) == "fLaC":
 		return ReadFLACTags(r)