Refactoring: Extract Dsl classes

Before Rails integration, I want to merge these refactoring.

## Old Plan was:

- Define SEO contents in `config/initializers/seroy/*`, can be extracted some files.
- The page contents definitions are reload on development mode.
- With the definitions and `include Seory::RailsHelper` in `ApplicationHelper` provides `serory()` method to get well managed `Seory::Runtime` instance.

After above, release '0.1.0'