WkHtmlToPdf now supports header/footer (simple)

Added helpers to facilitate easy header/footer and from View/Layout configuration of WkHtmlToPdf creation.
I was hoping the `debug()` would help developers figure out the non-intuitive setup required for creating the directory to contain the temp HTML.  

I commented it out for now, but we could strip completely... I just don't want to strand developers not knowing what to do... nor do I want to render to the screen (in non-dev) any error which might communicate paths on the server.