Every tag should have a slug
Fixes #18

 * Alter tags table to ensure name and slug can not be NULL
 * Validate presence of tag name and slug
 * Overload FriendlyId::Slugged#normalize_friendly_id to ensure a slug is set
 * If a generated slug is empty, use FriendlyId sequence separator as a default
Merge pull request #19 from simeonwillbanks/generate-slugs-for-all-tags

Every tag should have a slug