Add a safer settings / localStorage check.

This keeps blowing up in my current Disconnect extension in Chrome (Version `5.13.0`). When localStorage is not available a JavaScript error is thrown:

    Uncaught SecurityError: Failed to read the 'localStorage'
    property from 'Window': Access is denied for this document.

I'm not sure how to properly fall back on options, but without testing the extension in its entirety this is the best I have for now.

![screen shot 2014-02-26 at 8 28 25 pm](https://f.cloud.github.com/assets/1817/2279214/ba05cf34-9f67-11e3-8a1f-f5efcd2c5fcc.png)

![screen shot 2014-02-26 at 8 28 48 pm](https://f.cloud.github.com/assets/1817/2279215/bd6c8c30-9f67-11e3-9ee9-48afb49cd442.png)