Repair error of to_param when no slug column

[wbreeze]

This is to repair issue 20

It adds two tests for the to_param instance method:

• one which ensures override when there is a slug
• the other ensures no change in behavior on models without a slug

The fix proposed and imlemented here is to extend models with the slug instance methods only when slug is invoked at the class level.

[wbreeze]

The diff on this file is much clearer when viewed without the white space changes. We indented several methods within a module.

[bkoski]

This is a great improvement, thank you! (and thanks for the context in the PR)