build: wreck should use project install defs

We predefine the flux-core install directories in configure.ac so these
definitions needed not be repeated throughout the project.  Somehow we
missed converting wreck to use these and so wreck did not pick up
the redefinition of fluxmoddir in the 'submodule' patch series.
build: revert addition of cmb to fluxmoddir

Extra directory was added when module names were prefixed with "cmb."
but since that change did not make the final cut, we should revert
this change too.

Trivial to make this change again later, epecially since module loading
code now searches directories recursively, so it doesn't matter to
the loading code where in $fluxmoddir the modules live.
doc: update README.md and add overview section

It seemed like our README.md was a bit sparse.