Pagination upgrades

* Exposed some properties in Page that weren't previously available.
* Folded in jekyll-pagination to support non-Index.html pagination and allowing for paginating files in sub-directories (e.g. /blog/index.html)
  * Defaults to index.html for backward compatibility.
* Allow for customizing the pagination url style (e.g. /blog/page/:page => /blog/page/1).
  * Defaults to page:page (page#) for backward compatibility.

Updated version of #521.