fix(localStorageWrapper): fix early resolve of setItem for Blobs

Fix the early resolution of the setItem promise in case a Blob is used with localStorageDriver.
Wraps the _serialize call found in setItem method of localStorageDriver with a Promise.

Fixes #317.
Because of https://github.com/ariya/phantomjs/issues/11013 the Blob test does not currently run on PhantomJS. This will be fixed in PhantomJS v2.0. As a result, I used grunt serve + Firefox + Chrome to test on actual browsers. [Here](http://plnkr.co/edit/FA23NrkysU5prFwj0Yaj?p=preview) is a plunker based on the on provided by @ocombe in #317.
Okay. I think I should focus on fixing the saucelabs tests then, so we can
run these tests on actual browsers. I can probably make that a holiday
project :smile: