Add SystemXHR as a solution for whitelist in FFOS

Proposed content

In Firefox OS there is no concept of whitelisting a specific domain. Instead
there is a special permission called
[SystemXHR](https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest#Permissions).
There is a need to add this permission to `config.xml`:

<platform name="firefoxos">
<permission name="systemXHR" privileged="true" description="load data from server" />
</platform>

The `XMLHttpRequest` object needs to be instantiated with two parameters
`mozAnon` and `mozSystem`:

var request = new XMLHttpRequest(

{ mozAnon: true, mozSystem: true}

);

This solution is transparent so there is no difference for other platforms.