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
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
Rocket style hash and switching integers for Logger severity constants
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
Fixing help command in deprecation warning
Keep parents of directories in keep_files

Closes #1297
Merge pull request #2457 from marcransome/master
Update history to reflect merge of #2457 [ci skip]
Remove `Command.globs` method

It is no longer being used.
Get relative paths of directories to ignore

Use `Pathname#realpath` to get absolute paths so that
`Pathname#relative_path_from` will not raise an exception. Also add the
config files to this list.
Add jekyll_github_sample plugin
Merge pull request #2463 from bwillis/patch-1
Update history to reflect merge of #2463 [ci skip]
Using implicit returns to conform to coding style