Add Content::content_type registration.

Current spec status: 721 examples, 0 failures in 7.4 seconds.
Add Content Type drop down for pages.

Current spec status: 724 examples, 0 failures in 7 seconds.
Implement MarkdownContent.

Current spec status: 732 examples, 0 failures in 6.89 seconds.
Update Content generation.

- Default to TextContent instead of Content.
- Render human-sensible content type name in drop down.

Current spec status: 740 examples, 0 failures in 6.69 seconds.
Implement Admin::Features::ContentsController#fields.

Current spec status: 746 examples, 0 failures in 7.69 seconds.
Merge branch 'master' into feature-content-types
Dynamically render Content fields for Page#new.

Current spec status: 746 examples, 0 failures in 7.1 seconds.
Dynamically render Content fields for Page#edit.

Current spec status: 748 examples, 0 failures in 6.96 seconds.
Persist compatible content data across content types.
Refactor content selection to CoffeeScript concern.
Remove spring preloader.

Spring was causing issues with caching polymorphic models, such as
Content and Feature types.

Current spec status: 748 examples, 0 failures in 8.17 seconds.
Auto-register Content using ::inherited hook.

Current spec status: 748 examples, 0 failures in 6.86 seconds.
Merge pull request #12 from sleepingkingstudios/feature-content-types

Implement Content polymorphism in views.