Test camelCase field names in CRUD integration tests.

The CRUD integration tests have also been cleaned up a bit.
Add support for ajax errors.

The commit relies on the error serialization that was added in Ember
Data v1.0.0-beta.12.

https://github.com/emberjs/data/commit/4884933497adb4894a3621ad8726cc63c7d910d2
Add unit tests for adapter ajaxError function.

Also:

* Modified ajaxError() to make the behaviour more explicit when the
  responseText can't be parsed.
* Improved comments.
Merge pull request #35 from benkonrath/ajax-errors

Add support for ajax errors.