[WIP] Uniqueness validation

@bcardarella this is a WIP that needs some cleanup and tests but curious to get your thoughts on how you feel about this direction.

I'd ultimately like to get the API to a point where you can just pass in something like:
```javascript
vaidations: {
  email: {
    uniqueness: true
  }
}
```
and have ember-validations build a default api endpoint url based on the model and property being validated. Right now you have to pass in an explicit url where the validation runs server-side. 

```javascript
vaidations: {
  email: {
    uniqueness: {
      url: 'http://localhost:3000/api/validations' 
    }
  }
}
```

I'd also like to eventually abstract this out into some kind of a remoteValidator that people can tap into if they have other remote validations that don't have to do with uniqueness.