Update mobile environment score

As suggested in #644, mobile platforms are not perfectly secure platform.

This pull request renames "Secure environment" to "Passable environment" for mobiles to avoid making too broad promises and to incentivize user to read the new text that contains a few advices.

Hardware wallet score is renamed from "Very secure environment" to "Secure environment"

![capture du 2014-12-05 15 11 29](https://cloud.githubusercontent.com/assets/3578089/5325351/116cb684-7cb7-11e4-8097-6e30e7550f23.png)