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.
Try item#to_liquid before looking at the data
Use `raise` instead of logging & aborting
Bump to the latest Liquid version, 2.6.1
Making `highlight` behave more like redcarpet

See issue 2510 for details.
Fixing tests.
Include files with a url which ends in / in the site.html_pages list

https://github.com/jekyll/jekyll-sitemap/issues/27
Merge file system categories with categories from YAML.

Fixes #2527
Add test for the concatenation of categories
Fixes highlight.rb to correctly parse list values
unit tests
unit tests
handles line numbers with more than 1 digit
Before copying file, explicitly remove the old one

On Windows, FileUtils.cp(path, dest_path) will fail with a Permission
Denied if the dest_path already exists and is read-only. People have
complained about this since at least 2008.

This lets `jekyll build` work without error in Windows when one or more
read-only files exist.

This change will accommodate users of Team Foundation, which uses the
read-only flag for for source control status. (dumb, I know, but but a
real problem)
nil parsed in yaml is just a string, should be explicit
Merge pull request #2536 from letuboy/master
Update history to reflect merge of #2536 [ci skip]
Merge pull request #2535 from simonsarris/master
Update history to reflect merge of #2535 [ci skip]
Merge pull request #2531 from jekyll/merge-post-categories