Cache the collection#all call to improve performance.

Fixes #132, fixes #137
Adding page URL to previewer page data.
Require 'forwardable' in sprockets (bugfix)
Actually search the rest of the document

Do what the comment says. The offset parameter to with_index simply sets
the index offset, not the iteration offset.

Also closes #154.
Merge pull request #167 from Stebalien/fix-sprockets-forwardable

Require 'forwardable' in sprockets plugin.
Merge pull request #168 from Stebalien/fix-summary-generation

with_index does not set the iteration offset.
Reorganize/fix the Sprockets Plugin

The compile function of the sprockets plugin was out of sync with the
rest of ruhoh and no longer worked. In the process of fixing it, I ended
up reogranizing the previewer and compiler code to match the layout of
Ruhoh's built-in compiler code as much as possible.
Merge pull request #169 from Stebalien/reorganize-sprockets-plugin

Reorganize/fix the Sprockets Plugin
Merge pull request #163 from lynnfaraday/2.1_paginator_preview_url

Adding page URL to previewer page data.
Render widgets within the full master page context.

Should fix #160 and fix #164.
Need to require the preview.rb file (at least for 1.9.2)