Place certs

Fixes #586

This extends the white-label customizations to include an entry TargetAction, `iOSaddCertificate`, which copies the entry's specified certificate file into the target project and then uses [node-xcode](https://github.com/alunny/node-xcode) to add the file as a project resource.

It will be easy to create new actions, like including icon and splash screen assets in the project, by creating new javascript functions to implement the action and including a TargetAction name for those actions in the targetActions customize.js dictionary.

We need to document the new white-label automation features - see #594.