Added try/catch to avoid errors when cookies/storage is disabled

When trying to bootstrap the angular app with cookies/storage turned off, `var webStorage = $window[storageType]` was throwing an error. This caused the Angular app to fail bootstrap/init. This is similar to pull request 59 (https://github.com/gsklee/ngStorage/pull/59), but adds a shim `webStorage` object. It also adds a check for an error that I got in Firefox where `webStorage = $window['sessionStorage']` evaluated, but then calling `webStorage.length` threw a 'security' error.

Thank you for making ngStorage!