document the fixture adapter for ember data

Added some documentation on usage of the Fixture Adapter with an example
of subclassing DS.Model.
Update source/guides/models/the-fixture-adapter.md

Fixed FixtureStore example to be more consistent with Ember's naming conventions.
Adding data-field.com to the ember users
Resizing data-field.com logo to fix properly
Merge pull request #350 from nicolasembleton/master

Adding data-field.com in the Ember.js users
Merge pull request #228 from fivetanley/document-fixture-adapter

document the fixture adapter for ember data
missing DS in defining-models code example
remove confusing var with DS.attr
Merge pull request #351 from svanderbleek/attr-typo

missing DS in defining-models code example
Update rendering-a-template.md

fix typo
Include yaml requirement for Ruby 2.0
Document key, namespace, url, transform customization
Merge pull request #353 from mixonic/rest-model-customizations

Document key, namespace, url, and transform customizations
Merge pull request #352 from craigteegarden/patch-1

Update rendering-a-template.md