$cordovaPreferences: adding support for a preferences plugin #83

hi,

i have added support for a preferences plugin and create a preferences page in the demo page as well:

![image](https://cloud.githubusercontent.com/assets/62456/3316275/2bb3843c-f701-11e3-8acc-e9b6c8c62414.png)

let me know what you think ?

//

pelle