func NewDocument(title, rootTagName []byte) *Document {
	var image Document
	image.title = []byte(title)
	rootTag := NewTag([]byte(rootTagName))
	image.root = rootTag
	return &image
}
NewDocument creates a new XML/HTML/SVG image, with a root tag.
If rootTagName contains "<" or ">", it can be used for preceding declarations,
like <!DOCTYPE html> or <?xml version=\"1.0\"?>.
Returns a pointer to a Document.
package tinysvg
import (
	"bytes"
	"fmt"
	"io/ioutil"
)
type Tag struct {
	name        []byte
	content     []byte
	lastContent []byte
	xmlContent  []byte
	attrs       map[string][]byte
	nextSibling *Tag  
	firstChild  *Tag  
}
type Document struct {
	title []byte
	root  *Tag
}