Allow providing rule value as a function and add support for anchor custom type context.

There are 2 distinct commits in this PR that are related but do pretty different things.

**The first one lets you specify rule values as functions that have the currently set values as `this` context.** What does that mean? Here's a simple example:

```javascript
attributes: {
  startDate: {
    type: 'date',
    before: function() {
      return this.endDate;
    }
  },
  endDate: {
    type: 'date',
    after: function() {
      return this.startDate;
    }
  }
}
```

It also supports async functions:

```javascript
attributes: {
  website: {
    type: 'string',
    contains: function(cb) {
      setTimeout(function() {
        cb('http://');
      }, 1);
    }
  }
}
```

Note: the above is completely independent from anchor.
Note: the 3rd commit simply refactors my initial test to make more sense.


**The second commit provides the model's currently set values as the `this` context to anchor**. See https://github.com/balderdashy/anchor/pull/24 for more details. 

Please note: the second commit requires the aforementioned feature in anchor to be merged in, otherwise the tests will fail, even though it does not break any of the existing functionality.