Add default page margins

@@ -3,6 +3,7 @@
 * Change PdfColor.fromInt to const constructor
 * Fix drawShape Bézier curves
 * Add arcs to SVG drawShape
+* Add default page margins
 
 # 1.1.0
 * Rename classes to satisfy Dart conventions

@@ -19,11 +19,11 @@
 part of pdf;
 
 class PdfPageFormat {
-  static const a4 = PdfPageFormat(595.28, 841.89);
-  static const a3 = PdfPageFormat(841.89, 1190.55);
-  static const a5 = PdfPageFormat(420.94, 595.28);
-  static const letter = PdfPageFormat(612.0, 792.0);
-  static const legal = PdfPageFormat(612.0, 1008.0);
+  static const a3 = PdfPageFormat(29.7 * cm, 42 * cm, marginAll: 2.0 * cm);
+  static const a4 = PdfPageFormat(21.0 * cm, 29.7 * cm, marginAll: 2.0 * cm);
+  static const a5 = PdfPageFormat(14.8 * cm, 21.0 * cm, marginAll: 2.0 * cm);
+  static const letter = PdfPageFormat(8.5 * inch, 11.0 * inch, marginAll: inch);
+  static const legal = PdfPageFormat(8.5 * inch, 14.0 * inch, marginAll: inch);
 
   static const point = 1.0;
   static const inch = 72.0;
@@ -33,15 +33,43 @@ class PdfPageFormat {
   final double width;
   final double height;
 
-  const PdfPageFormat(this.width, this.height);
+  final double marginTop;
+  final double marginBottom;
+  final double marginLeft;
+  final double marginRight;
+
+  const PdfPageFormat(this.width, this.height,
+      {double marginTop = 0.0,
+      double marginBottom = 0.0,
+      double marginLeft = 0.0,
+      double marginRight = 0.0,
+      double marginAll})
+      : marginTop = marginAll ?? marginTop,
+        marginBottom = marginAll ?? marginBottom,
+        marginLeft = marginAll ?? marginLeft,
+        marginRight = marginAll ?? marginRight;
+
+  PdfPageFormat copyWith(
+      {double width,
+      double height,
+      double marginTop,
+      double marginBottom,
+      double marginLeft,
+      double marginRight}) {
+    return PdfPageFormat(width ?? this.width, height ?? this.height,
+        marginTop: marginTop ?? this.marginTop,
+        marginBottom: marginBottom ?? this.marginBottom,
+        marginLeft: marginLeft ?? this.marginLeft,
+        marginRight: marginRight ?? this.marginRight);
+  }
 
   PdfPoint get dimension => PdfPoint(width, height);
 
   PdfPageFormat get landscape =>
-      width >= height ? this : PdfPageFormat(height, width);
+      width >= height ? this : copyWith(width: height, height: width);
 
   PdfPageFormat get portrait =>
-      height >= width ? this : PdfPageFormat(height, width);
+      height >= width ? this : copyWith(width: height, height: width);
 
   @override
   String toString() {