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.

id3v2.go

@@ -7,7 +7,6 @@ package tag
 import (
 	"fmt"
 	"io"
-	"os"
 	"strconv"
 	"strings"
 )
@@ -302,11 +301,6 @@ func (r *unsynchroniser) Read(p []byte) (int, error) {
 // ReadID3v2Tags parses ID3v2.{2,3,4} tags from the io.ReadSeeker into a Metadata, returning
 // non-nil error on failure.
 func ReadID3v2Tags(r io.ReadSeeker) (Metadata, error) {
-	_, err := r.Seek(0, os.SEEK_SET)
-	if err != nil {
-		return nil, err
-	}
-
 	h, err := readID3v2Header(r)
 	if err != nil {
 		return nil, err