Add the ability to customize properties in object inspector

By adding a `_debugInfo` method to an object, we can now customize how the properties will appear in the object inspector.

There will be a PR in parallel to Ember Data to add `_debugInfo` to `DS.Model`.

Example with Ember Data:

![extension_screenshot](https://f.cloud.github.com/assets/1061742/903690/5a951104-fbb1-11e2-9c5a-ae736c119ca5.png)



`_debugInfo` returned value:

```javascript

propertyInfo: {
  // include other mixins/properties not listed here
  includeOtherProperties: true,
  // do not display these properties
  skipProperties: [],
  // do not display these mixins along with their properties
  skipMixins: [],
  // don't calculate these CPs automatically (unless cached)
  expensiveProperties: [
    'businessLine', 
    'organization',
    'linkedOrganization', 
    'planGroupsProducts', 
    'productConditionsClauses'
  ],
  groups: [
    {
      name: 'Attributes',
      properties: ['id', 'code', 'name' ],
      expand: true // open by default
    },
    {
      name: 'Belongs To',
      properties: [ 'businessLine', 'organization', 'linkedOrganization' ],
      expand: true
    },
    {
      name: 'Has Many',
      properties: [ 'planGroupsProducts', 'productConditionClauses' ],
      expand: true
    },
    {
      name: 'Flags',
      properties: ['isLoaded', 'isDirty', 'isSaving', 'isDeleted', 'isError', 'isNew', 'isValid']
    }
   ]
  }
 ```