Rename Mandel::Description to ::Model
Add Mandel->model()

  - Add "collection_class" attribute to Mandel::Model
  - Add "connection" attribute on Mandel::Collection
  - Change "model" attribute in Mandel::Collection to "connection"
  - Change "model" attribute in Mandel::Document to "connection"
  - Remove "document_class" from Mandel::Collection