Beefs up variables.md

Adds site.data
Adds site.collections
Adds site.html_pages
add collection.label to Document#to_liquid
add label to document section of collections page
Take advantage of date_to_rfc822

Jekyll has a filter built just for this purpose
Spice up site_template feed
Update explanation of the site front-matter defaults section
Allow Jekyll's logger to be set to any Logger compatible instance

* Stevenson now inherits from Logger and extends some methods to use $stderr
for log messages greater than info level.
* LogWriter provides an interface between Jekyll and Logger to maintain API.
Renaming LogWriter to LogAdapter and adding tests for class

String coloring moved to Stevenson as not responsibility of LogAdapter
Update link to "Migrate" in README
Add link to #jekyll on irc.freenode.net
Rocket style hash and switching integers for Logger severity constants
Output Jekyll output if ENV['DEBUG'] exists
Add the paginator as a plugin: jekyll-paginate
Bump to jekyll-paginate 1.0.0
Merge pull request #2455 from jekyll/extract-pagination-generator
Update history to reflect merge of #2455 [ci skip]
Merge pull request #2437 from pathawks/date_to_rfc822
Update history to reflect merge of #2437 [ci skip]
Tweak front-matter vs site configuration per comments from @parkr on jekyll/jekyll#2439
Make sure Cleaner doesn't remove dirs if they only contain subdirs

Closes #2204