Alias "int" for NumberTransform

Myself and a few others starting with Ember run into confusion when trying to use `DS.attr('int')` or `DS.attr('integer')`. While it's stated in the documentation that there's a `number` attribute, it may be helpful to have something like this for such a common error.

If not an alias, then perhaps a warning/error for undefined `DS.attr()` in debug mode? 

I'm not very familiar with the codebase, and I can see this getting rejected for many reasons =) 

Related: https://github.com/emberjs/data/issues/23