Add page_description helper, refactor internals

* Use isolated testing style, not dummy app in spec suite.
* Add `page_description` helper similar to `page_title`.
* Refactor internals to have a single `Flutie::ViewHelper`.
* Have `page_title` and `page_description` use i18n keys instead of
  `content_for`.
* Document changes.

This is basically a complete rewrite and would introduce breaking changes for the `page_title` helper. I imagine we'd release a 3.0.0 version.