Expose Jekyll version to templates

Make available `site.version` to liquid
Use proper Ruby syntax for constant
Merge branch 'patch-1' of git://github.com/pathawks/jekyll into pathawks-patch-1

* 'patch-1' of git://github.com/pathawks/jekyll:
  Use proper Ruby syntax for constant
  Expose Jekyll version to templates
Put version under 'jekyll' namespace.
Add cucumber test for `jekyll.version`.
Add Good Include to plugin list
Fix include tag to avoid NameError
Remove kramdown as a gem dependency
Add kramdown as a development dependency
Merge pull request #1491 from penibelst/list-plugin-good-include

Add Good Include to plugin list
Update history to reflect merge of #1491
XML-escape blog post title in feed.xml
Added link to TOC Generator Filter Plugin
Merge pull request #1501 from dpree/patch-1

XML-escape blog post title in feed.xml
Update history to reflect merge of #1501
Merge pull request #1498 from troyswanson/remove-kramdown-as-dependency

Remove kramdown as a gem dependency
Update history to reflect merge of #1498
Merge pull request #1494 from maul-esel/fix-include-error

Fix include tag to avoid NameError
Update history to reflect merge of #1494
Merge pull request #1481 from mojombo/pathawks-patch-1

Expose Jekyll version to templates: `jekyll.version`