Allow pdfConfig to be altered from the PdfView

base level changes needed for PDF config being changable from the View level
(helper methods in the next commit)
Helper methods from PdfView View to set pdfConfig

You can now call `$this->pdfOptions()` and other helpers
from any View to set options for CakePdf / wkhtmltopdf

You can also call any of these helpers:

* $this->pdfHeader($key, $content) $config.options.header-<key>
* $this->pdfFooter($key, $content) $config.options.footer-<key>
* $this->pdfOptions($options)      $config.options (for wkhtmltopdf)
* $this->pdfConfig($config)        $config
WkHtmlToPdfEngine header/footer html handler

You can pass in any HTML you want for the header/footer,
but WkHtmlToPdf needs it to be in a stand-alone HTML file.

This handler will create that file and wrap your content.

    input content: <p>Some HTML here</p>
    output: pdf-temp/header-html-52bf266917d266accbb0b794fae83062.html
    folder: WWW_ROOT . 'pdf-temp'
    path: WWW_ROOT . 'pdf-temp/header-html-52bf266917d266accbb0b794fae83062.html'

Config:

* `webroot-temp-folder` (string) name of folder in webroot [default: `pdf-temp`]
* `webroot-temp-as-full-url` (boolean) if true, use full URL when requesting [default: `false`]
* `webroot-temp-disable-wrapper` (boolean) if true, we will not wrap content block
Updated README w/ WkHtmlToPdf options/example

Since we now support header/footer on WkHtmlToPdf,
we might as well extend our examples to demonstrate how much we can do with it.