Rename Page.printToPDF into page.pdf

This patch:
- renames Page.printToPDF into page.pdf
- adds a 'path' option to the page.pdf options instead of a separate
  `filePath` parameter
- improves on the documentation for the `page.pdf`

References #39.

lib/Page.js

@@ -422,9 +422,9 @@ class Page extends EventEmitter {
   /**
    * @param {string} filePath
    * @param {!Object=} options
-   * @return {!Promise}
+   * @return {!Promise<!Buffer>}
    */
-  async printToPDF(filePath, options) {
+  async pdf(options) {
     options = options || {};
 
     let scale = options.scale || 1;
@@ -465,7 +465,9 @@ class Page extends EventEmitter {
       pageRanges: pageRanges
     });
     let buffer = new Buffer(result.data, 'base64');
-    fs.writeFileSync(filePath, buffer);
+    if (options.path)
+      fs.writeFileSync(options.path, buffer);
+    return buffer;
   }
 
   /**
@@ -652,7 +654,7 @@ function convertPrintParameterToInches(parameter) {
     console.assert(!isNaN(value), 'Failed to parse parameter value: ' + text);
     pixels = value * unitToPixels[unit];
   } else {
-    throw new Error('printToPDF Cannot handle parameter type: ' + (typeof parameter));
+    throw new Error('page.pdf() Cannot handle parameter type: ' + (typeof parameter));
   }
   return pixels / 96;
 }