First pass at adding validations to Ember-Model.

Hi folks.

Here's my first go at adding validations to Ember-Model.  It's done by way of adding a `Ember.Valadatable` mixin, which allows users to define `fooValidator` or `fooValidators` properties on their models, and pass in subclasses of `Ember.Validation`, which have a very simple API for writing validations.

This is my first pass at doing this, please don't be shy in giving feedback.