Add stubs for some more eta facts
Add proof of eta-postponement!
Add proofs about eta's
Add proof that eta_par is substitutive

Final `admit` (except for the lift / subst lemmas) eliminated!
Prove last admitted lemmas
Fix and prove lift_subst_semicom
Rename lift_subst_semicom into lift_distr_subst

Because that's what it is.
Merge pull request #20 from knuton/final_touches

Proof of Postponement theorem and remove all admits