Implement Tagging model with polymorphic Taggable assoc.

Current spec status:
- Jasmine: 31 specs, 0 failures in 0.035s.
- RSpec: 283 examples, 0 failures in 3.6 seconds.
Add Taggings to BlogPost form, show, preview actions.

Current spec status:
- Jasmine: 31 specs, 0 failures in 0.043s.
- RSpec: 297 examples, 0 failures in 3.71 seconds.
Implement BlogTagsController, show action.

Current spec status: 307 examples, 0 failures in 3.69 seconds.
Merge pull request #17 from sleepingkingstudios/feature-taggings

Implement polymorphic Tagging model.