LocalStorage fix

Fixes #65 

PoC in the image attached:

As you can see, the first two windows show iPhone sessions that reached the localStorage quota (localStore is empty), the staging instance (2nd window, with applied patch) doesn't throw the error.

![screen shot 2014-03-12 at 5 50 00 pm](https://f.cloud.github.com/assets/705860/2404722/48c13e00-aa41-11e3-862f-7b623eb7eadb.png)

Code borrowed from https://github.com/Modernizr/Modernizr/blob/master/feature-detects/storage/localstorage.js