Added supprt for iOS / Android native social login / native share for Facebook and Google +

To make native login work as expected, fill the files :
Android:
Assets/Plugins/Android/bridge/strings.xml (set the app_id to your facebook app id)
iOS:
Assets/Plugins/Scoreflex/Info.plist
  Set FacebookAppId, FacebookAppName, value of the url for facebook is fb[appId];
  Set GooglePlusAppId and the package names in the url