Make redcarpet a runtime dependency
Update the QuickStart guide for the default engine change

As of Jekyll 1.1, we'll be using Redcarpet as the default for all new
sites.
Merge pull request #1247 from mojombo/jekyll-new-redcarpet

Complete the transition to Redcarpet for new sites
Update history to reflect merge of #1247.
Add site.pages to Variables page in Docs. Fixes #1251
Update history to reflect merge of #1251.
Add Youku and Tudou Embed link on Plugins page in site/. Fixes #1250.
Update history to reflect closing of #1250.
Add note that gist tag supports private gists. Fixes #1248.
Update history to reflect closing of #1248.