Schema-103. Hide ColumnNameTranslator.

[The link for the associated ticket](https://jira.kiji.org/browse/SCHEMA-103)

I marked this as private and moved it to the layout.impl package. This class gets instantiated and used all over the codebase, so package protecting the constructor won't work for cordoning this off more. Is a factory in order, or will this "hiding" of the class suffice?