isDirty fixes

* fix isDirty breaking when defining more than one attribute in `create`
* materializing relationships should not dirty record
Put record in recordCache before calling load on it

Sometimes you may want to override load to extend the way it works. In
such case it will fail when you use any function which uses recordCache,
because it will call load again and you will end up with an infinite
recursion. This commit fixes the situation by putting the record in
recordCache before running load on it.
Merge branch 'fix-attr-data'

* fix-attr-data:
  isDirty fixes
Give the new tests some more descriptive error messages.
Customize error message for Ember.Adapter

https://github.com/ebryn/ember-model/issues/118

Added class name to error messages.
Merge pull request #169 from seeweer/issue_118

Customize error message for Ember.Adapter
Added posibility to find record with id 0.

https://github.com/ebryn/ember-model/issues/156

There was a small bug on buildURL method. When id was 0 buildURL skipped creating url with id.
Merge pull request #170 from seeweer/issue_156

Added posibility to find record with id 0.