Added tutorials and refs in code comments

Author: Filippo Conti

src/ast.js

@@ -104,6 +104,7 @@ const Position = require("./ast/position");
 /**
  * The AST builder class
  * @constructor AST
+ * @tutorial AST
  * @property {Boolean} withPositions - Should locate any node (by default false)
  * @property {Boolean} withSource - Should extract the node original code (by default false)
  */

src/index.js

@@ -38,7 +38,7 @@ function combine(src, to) {
  * Initialise a new parser instance with the specified options
  *
  * @class
- *
+ * @tutorial Engine
  * @example
  * var parser = require('php-parser');
  * var instance = new parser({

src/parser.js

@@ -16,6 +16,7 @@ function isNumber(n) {
  * The PHP Parser class that build the AST tree from the lexer
  *
  * @class
+ * @tutorial Parser
  * @property {Lexer} lexer - current lexer instance
  * @property {AST} ast - the AST factory instance
  * @property {Integer|String} token - current token