Manifest updates and details on installing

I've published the extension in the Chrome Web store: https://chrome.google.com/webstore/detail/planet-view/hhhgdbldiopbpblfcohjaeinjjciplho

This updates the version number, allows use in incognito mode (requires a preference setting on the extensions page), and updates the installation instructions.

![planet-view-install](https://cloud.githubusercontent.com/assets/41094/5060900/8fa2e39a-6d29-11e4-86ef-1f9f95ed0cd0.gif)

Fixes #5.