Refactor into OOP and implement Thor

This commit moves functionality for generating pages into a Thor script
and helper module. It introduces a PostPage, Symlinker, and Editor
Class. See the attributes and public functions that define the public interface.

This change makes use of a more robust command line solution (thor) and
makes the solution more maintainable by introducing an oop design.

* The Thorfile requires Thor to run. (gem install thor). It provides two
tasks, `Thor page:new TITLE CATEGORY` and `Thor page:relink`. To see all
the tasks and their descriptions, run `Thor list` in the terminal.
* A few notes on the page:new task- it now requires both title and
* category. Before they were optional. Also, Thor page:relink deletes all the files in _pages and regenerates them.
* The lib/helpers.rb file provides a Helpers module and a Page class. This
module could be expanded to include other helper classes.