Adding BgImage Class

This class does all the heavy lifting and gives us our CSS to use in our
<head>
Sample <head>

The header which instantiates a new instance of the BgImage class, then
runs the getBgImageCSS() method to retrieve the styling we'll need.
Background_Image content template file

This file should live in 'perch\templates\content'. It is set to
'display:none' and has 'suppress="true"' on all perch elements as we're
not wanting to output anything here, but rather tell the Perch CMS that
this page has a background image required (with background position
options).