Move a section later

The notion of projective dimension needs to be defined in order to be
able to talk about it
Clarify a lemma in derived.tex
Beautify a formula in hommology.tex
Skeleton sections on pro- and injective dimension
\text{Coh} -> \textit{Coh}
Internal hom in D(R)

It is a good day when the sign conventions work out on the nose! Woohoo!