Merge pull request #1306 from robwierzbowski/add-jit-plugin

Add Jekyll Image Tag to the plugins list
Update history to reflect merge of #1306.
Update history page in the docs to include #1304.
Merge branch 'master' of github.com:mojombo/jekyll

* 'master' of github.com:mojombo/jekyll:
  Add Jekyll Image Tag to the plugins list
Organize plugins into categories, clean up copy.
Escape/remove single '_' and '*'s that maruku chokes on.