Transition missed _config.yml changes to new _config files

With the transition to the new "_config" directory structure, some
of the changes from _config.yml were missed. Integrating all changes
based on _config.yml@9659a45.

* _config/defaults/classic.yml: Add missed blog_index_sidebar,
  page_sidebar, post_sidebar entries; remove duplicate excerpt_link
  entry.
* _config/defaults/disqus.yml: Add missed disqus_developer entry.
* _config/defaults/google_plus.yml: Add missed
  google_plus_image_size entry.
* _config/defaults/jekyll.yml: Add missed include entry; add missed
  markdown=redcarpet and supporting entries.
* _config/defaults/sharing_buttons.yml: Add missed
  respectfully_social entry.
full_configuration in Rakefile was uninitialized

Switched to a variable instead and reusing it in `integrate` task as well.
Fixed configuration overriding

The deep merge was made the wrong way around
Added initial test coverage
Merge pull request #1021 from gkarekinian/patch-1

full_configuration in Rakefile was uninitialized
Upgrade history to reflect merge of #1021.
Merge pull request #1022 from gkarekinian/patch-2

Fixed configuration overriding
Ruby development console.
Octopress::Configuration is now a class, rather than a module.

The previous system was very unforgiving about where the configuration directory
lay. This commit allows the configuration directory to be set through a config_directory
instance variable on the Octopress::Configuration class, in the event that this directory
ever needs to be changed (e.g. for testing).
Updated Rakefile to handle Octopress::Configuration as a class.
Specifying an exact version for rdiscount.
Testing now in lib/, added a supporting Gemfile and Rakefile
Merge pull request #1001 from tonyduckles/new-configs-fixes

Transition missed _config.yml changes to new _config files