Add some auxiliary methods to Model

* Added memberUri, which returns RESTful URL for this resource as a
  member, like http://www.example.com/path/to/post/1
* Added collectionUri, which returns RESTful URL for this resource as a
  collection, like http://www.example.com/path/to/posts
Merge pull request #5 from astashov/add-some-aux-methods

Add some auxiliary methods to Model
Add some small tweaks

* Fixed warning for queryParameters in changeable_uri.dart
* Fixed returning type in model_association.dart
* Don't send GET requests as "application/json" to avoid OPTIONS requests