Implement page.waitForFunction method

The page.waitForFunction method allows to wait for a general predicate.
The predicate will be continiously polled for in page, until
it either returns true or the timeout happens.

The polling parameter could be one of the following:
- 'raf' - to poll on every animation frame
- 'mutation' - to poll on every dom mutation
- <number> - to poll every X milliseconds

References #91

lib/helper.js

@@ -23,7 +23,7 @@ class Helper {
    * @return {string}
    */
   static evaluationString(fun, ...args) {
-    if (typeof fun === 'string') {
+    if (Helper.isString(fun)) {
       console.assert(args.length === 0, 'Cannot evaluate a string with arguments');
       return fun;
     }
@@ -181,6 +181,22 @@ class Helper {
   static recordPublicAPICoverage() {
     apiCoverage = new Map();
   }
+
+  /**
+   * @param {!Object} obj
+   * @return {boolean}
+   */
+  static isString(obj) {
+    return typeof obj === 'string' || obj instanceof String;
+  }
+
+  /**
+   * @param {!Object} obj
+   * @return {boolean}
+   */
+  static isNumber(obj) {
+    return typeof obj === 'number' || obj instanceof Number;
+  }
 }
 
 module.exports = Helper;