issue #118: Allow multiple relationships to same model

[chieffancypants]

Just required another loop

[jmdobry]

Taking another look at it, the new implementation here could be ambiguous, and even allow the same relation to be defined multiple times. Also, I don't really like having arrays of objects with each object only having one key/value pair. I think it should be modified to the following:

belongsTo: {
  user: [
    {
      localField: 'created_by',
      localKey: 'userId'
    },
    {
      localField: 'modified_by',
      localKey: 'userId'
    }
  ]
}

I moved the array one level deeper.

Also, the array check should use DS.utils.isArray to be as cross-browser as possible.

[chieffancypants]

Sure, that makes more sense. I force pushed over the previous commit

[jmdobry]

Sweet, I'll be looking at this soon.