Unfortunately there's no CameraCaptureUI.CaptureFileAsync() in Windows Phone 8.1 so Universal Apps which use this method on Windows 8.1 can't share the code with Windows Phone 8.1.
I wrote a little helper control that fills this gap for the demo apps I'm using. When you reference this project from your Windows Phone project and add the namespace DMX.Helper you can use CameraCaptureUI.CaptureFileAsync for taking Pictures.
There aren't any settings supported yet, also video recording isn't supported yet, but you could integrate those functions when you adjust the code.
Go here for Details:
This is a demo Project. Don't use in production. No support.