New chapter about differential graded algebra

Short term goal: develop enough theory so we can use it to prove the
result that D_{QCoh}(X) = D(A) for some differential graded ring A
when X is a qc+qs algebraic space
Preparing to prove homotopy category is triangulated

It is _utterly_ the same as for the homotopy category of
complexes in an additive category.
The homotopy category is a triangulated category

Easy peasy lemon squeezy!
Good resolutions

Just wonderful.
Perfect is compact in qc + qs case

Very general. Of course often there are more compact objects
than perfect objects.
Compact = Perfect in D_{QCoh}(X) for schemes and spaces

Easy peasy lemon squeezy!
Derived category and delta functor
Differential graded categories

Just the definitions and two examples
Work with right modules, of course!

Sorry Mike!
Restriction and base change

This is quite ugly at the moment.
Projective and injective graded modules

Fun!
Correct way to compute RHom(N, -) in DGA setting
Mainly definitions of dga functors, etc
Adjointness of tensor + hom and application

The application is that if you have a compact object K of
D(O) where O is a sheaf of rings on a ringed site, then
you get an embedding of D(E) into D(O) where E is the dga
of endomorphisms of K.

Still a bit rough here and there.
Here we get D(E) = D_{QCoh}(X) for X qc + qs
Move dga chapter later

Currently we use ringed sites and K-injectives. We should probably
move that out of the chapter and then move it earlier again.
Here we get D(E) = D_{QCoh}(X) for X qc + qs algebraic space

Great!
Merge branch 'develop'

This merges the material on differential graded algebras

We do the absolute minimum of work in order to be able to
prove the result of Bondal and van den Bergh that D_{QCoh}(X)
is equal to D(E) when X is a qc + qs scheme or space.

But it is clear that the language of differential graded
modules and categories could be used in many more places.