[iOS9] ability to set NSAppTransportSecurity settings from manifest

[collingreen]

ios9 requires either whitelisting remote urls that will be accessed or disabling the security altogether. Each devkit build wipes out the previous plist file, so hand editing it for these keys is not practical.

[collingreen]

This is possible with the existing plist flag in config.json.

For reference, this entry in config.json will disable the NSAppTransportSecurity issues (this is the sledgehammer-hack way - ideally each domain will be whitelisted instead of simply disabling the security altogether).

  "plist": {
    "NSAppTransportSecurity": {
      "NSAllowArbitraryLoads": true
    }
  }