Add ::slugify_all! class helper to Sluggable.

Iterates through collection and updates slug values. Useful when adding
concern to a model with existing documents.

Current spec status: 429 examples, 0 failures in 1.63 seconds.
Fix edge case for empty string slugs.

Update documentation.

Current spec status: 431 examples, 0 failures in 1.71 seconds.
Merge pull request #7 from sleepingkingstudios/feature-slugify-collection

Add ::slugify_all! class helper to Sluggable.