Beefs up variables.md

Adds site.data
Adds site.collections
Adds site.html_pages
Adds default front-matter support for collecitons
Includes tests for frontmatter defaults in documents
Passes the url to FrontmatterDefaults.all instead of full path
Adds the missing tests for #2405
Adds an example how to use frontmatter defaults with collections
Fixes the extra indentation in cucumber test
Skip initial build of site on serve with flag.

Adds --skip-initial-build flag to jekyll-serve, which serves the destination
immediately rather than waiting for the site to be built.

Fixes #1252.
Allow usage of different Twitter and Github usernames

and show these links only if twitter-username or github.username are set on _config.yml
Default entries for twitter/github usernames
Try item#to_liquid before looking at the data
Use `raise` instead of logging & aborting
Bump to the latest Liquid version, 2.6.1
Change Github/Twitter keywords to site.{twitter,github}_username
Re: https://github.com/jekyll/jekyll/pull/2485/files#r13525347
Merge pull request #2485 from tomer/patch-2
Update history to reflect merge of #2485 [ci skip]
Bump pygments.rb to 0.6.0.

Closes #2504.
Update history to reflect the closing of #2504. [ci skip]
Merge pull request #2419 from jens-na/collection-defaults
Update history to reflect merge of #2419 [ci skip]