Add slug field to BlogPost.

Automatically updated when the title is changed, unless a value has
been set manually.
Treat empty slug in form as slug_lock => false.

Current spec status: 270 examples, 0 failures in 3.07 seconds.
Use slug as blog post identifier on public-facing pages.

Current spec status: 270 examples, 0 failures in 2.94 seconds.
Update Mongoid dependency.

Current spec status: 270 examples, 0 failures in 3.31 seconds.
Merge pull request #15 from sleepingkingstudios/feature-blog-post-slug

Feature blog post slug