Use eso-pic for backgrounds

[BrianCriswell]

Switching to eso-pic for backgrounds provides the following advantages:

• It ensures that background images (such as the paper background) get placed behind other content.
• It also opens up the ability for users to more easily add their own background images and one-off backgrounds (like for \part).
• It provides a means for more interesting image options that can exceed the limits of the column or text area and spill behind or over other content.

[BrianCriswell]

#326