Add "createOnly" properties option

[jakerella]

I needed to be able to allow REST addition of properties on create, but disallow any update of those properties after creation. This PR implements a second value option for the "ReadOnly" mixin when specifying properties:

"mixins": {
  "ReadOnly": {
    "status": true,  // <-- ALWAYS read only from REST (all operations)
    "role": "createOnly"  // <-- can be set on creation from REST, but not updates from REST
  }
}

[prochac]

should by extended to whiteListed methods ex. ReadOnly: { email: { writableBy: ['create', 'prototype.patchAttributes'] } }