add event listeners, parse and stringify

isFn and isObj helper methods were added

SQUIZ.localstorage is set to true if the browser supports localstorage

SQUIZ.addEventListener and SQUIZ.removeEventListener functions were
added

SQUIZ.parse and SQUIZ.stringify were added (taken from json2.js) and
modified slightly, so they can be used internally for localstorage
functions

SQUIZ.local.set,
SQUIZ.local.get,
SQUIZ.local.remove
methods for localstorage were implemented

localstorage were added

todo:
- tests for set, get and remove methods

tests for parse and stringify methods aren't needed