Add command gitbook pdf to generate PDF

lib/generate/pdf/Index.js

@@ -17,7 +17,7 @@ var Generator = function() {
 
     // Options for PDF generation
     this.options = _.defaults(this.options, {
-        format: "A4"
+        paperformat: "A4"
     });
 };
 util.inherits(Generator, BaseGenerator);
@@ -34,7 +34,7 @@ Generator.prototype.finish = function() {
             "generate",
             path.join(that.options.output, "index.html"),
             path.join(that.options.output, "index.pdf"),
-            "--format="+that.options.format
+            "--format="+that.options.paperformat
         ].join(" ");
 
         exec(command, function (error, stdout, stderr) {