README improvements

* Made it so that `@nest` was just `nest`, to avoid confusion
* Removed screenshots of the Alfred plugin, since they were missing:

![screen shot 2014-12-17 at 5 04 56 pm](https://cloud.githubusercontent.com/assets/1724000/5479831/d91125a2-860e-11e4-92ef-56e49eb30a57.png)

https://nest.com/thermostat should be sufficient. I'm hoping that url is more resilient to changes.