  We define the notion of an additive model category, and we prove that any
additive, stable, combinatorial model category has a natural enrichment over
symmetric spectra based on simplicial abelian groups. As a consequence, every
object in such a model category has a naturally associated endomorphism ring
inside this spectra category. We establish the basic properties of this
enrichment.
  We also develop some enriched model category theory. In particular, we have a
notion of an adjoint pair of functors being a 'module' over another such pair.
Such things are called "adjoint modules". We develop the general theory of
these, and use them to prove a result about transporting enrichments over one
symmetric monoidal model category to a Quillen equivalent one.
