Fix to ember-validations to accept 'content' property

Hi Louis!

Thanks for writing this library! I've found it useful and incorporated it into my projects.

I made one small fix, where the current implementation doesn't like to nest errors inside any property that is also named 'content' -- this property conflicts with the ValidationError class's 'content' property. So I renamed the ValidationError's properties from 'content' and 'nestedErrors' to '_content' and '_nestedErrors'. And added a test.

Hope this helps!

-Gabe