Require that tool description documents say what tool schema version they are using.
Spec defines $ref, $mixin and $expr.  Decided $expr didn't need a "lang" field
and made it simpler by eliminating the inner object.  Replaced references to
"rabix" github organization with "common-workflow-language".  Updated schema
references to specifically refer to draft-1 branch.
Working on input schema section.
Working on inputs example section.
Finished 1st pass through tool description spec.
Move execution details to "execution" section to be explicit about the roles of
scratch space and output directory.
Incorporate runtime-environment.md into tool-description.md.  Update metaschema
to coorespond to draft 1 tool spec.  Delete contents of workflow spec document
to avoid misleading people into thinking there is something ready to use.