Allow idProperty to be defined as nested

[schallm]

I see that dstore is adding nested properties for queries (https://github.com/SitePen/dstore/commit/a822f92846024a8ac49254ada25e8ea63b62e5c3). It would be great if the idProperty could be nested as well. This would allow for the following:

new Memory({ idProperty: "referenceKey.id", data: response.expenditures })

To consume the following JSON:

{
    "expenditures": [
       {
         "name": "foo",
         "referenceKey": {
           "id": 250004,
           "version": 1
         },
         "auditInformation": {
           "enteredBy": "billy",
           "enteredDate": "2015-03-02T18:43:29"
         },
         "status": 1
       },
       {
         "name": "bar",
         "referenceKey": {
           "id": 250001,
           "version": 15
         }, 
         "auditInformation": {
           "enteredBy": "tommy",
           "enteredDate": "2015-03-15T10:23:47"
         },
         "status": 1
       }
     ]
}