Added Slugify mixin
Renamed Slugify concern to Sluggable, basic implementation

- current spec status: 13 examples, 0 failures
Added :lockable option to Sluggable::slugify

- current spec status: 19 examples, 0 failures
Fixed Sluggable locking error, added documentation

- current spec status: 20 examples, 0 failures
Updated version to 0.1.0