Basic support for validation errors on base

In Rails, there is this convention that when a validation error does not belong to any attribute, but rather to the record as a whole, it is assigned to the key "base". This pull requests adds very basic support for this feature to ember-data.

There is one question: when should the base errors be cleared? Attribute errors are cleared when the corresponding attribute changes, but there is no attribute "base". I decided to clear them when *any* attribute changes. Not sure if that is the right thing to do though.