Adds a resource for requesting custom post types

**Don't merge yet**

- [ ] the hardcoded `/posts` could be replaced with a placeholder

The WP API doesn't follow a strict RESTful approach to URLs. 
For example, to get posts from a specific post type, the URL looks like this:
`http://local.wordpress.dev/wp-json/posts/?type=my_post_type`

With the current implementation of a general resource for all requests, it's not possible to generate these URLs.
Here I introduce a separate resource for getting posts and passing a parameter for the post type:

```
$scope.posts = WPAPI.Posts.query( {
    type: 'my_post_type'
} );
```

As the `type` parameter has no placeholder, it's converted to a query argument.

Disclaimer: I'm an AngularJS noob, so this may not be the ideal approach :)
Maybe using the $http would be better as this API is not 100% RESTful?