Support for postgres uuid

[vespertilian]

It would be great if the loopback connector supported the postgresql UUID datatype for better performance.

http://simononsoftware.com/how-to-store-uuids-in-postgresql/

I tried to add it as a datatype but it gets set as numeric, when I think it should be mapped to a JS string.

[aol-nnov]

@vespertilian please refer to #54 It will give you some glue on how to use postgresql uuid data type

[vespertilian]

Thanks

@aol-nnov

  "id": {
      "type": "string",
      "id": true,
      "required": false,
      "defaultFn": "uuid",
      "postgresql": {
        "dataType": "uuid"
      }