Pjax API docs.

Refactor Pjax into pjax-base, pjax, and pjax-plugin modules. PjaxBase is meant to be a mixin for Controller. Pjax extends Controller and mixes in PjaxBase to provide a usable Pjax implementation. Plugin.Pjax extends Pjax and can be plugged into a Node instance.

Split pjax into "pjax" and "pjax-plugin" modules. Y.Pjax has all the functionality, but isn't a Node plugin so it doesn't actually do anything with Ajax responses. Y.Plugin.Pjax injects the response content into the host node and updates the page title.