Tidy up code / add comments for Eta
Tidy up Beta, add some comments
Rename eta subst lemmas and reintroduce eta_par_substitutive

Turns out it's needed after all!
Add a few auxillary lemmas about lam_k
Add proof of main lemma
Add to-be-proved lemmas on beta and beta_par
Add all necessary lemmas on the various beta closures

Work in progress, they should be moved to the Beta module
Move facts about beta to the Beta module
Merge pull request #18 from knuton/progress

Some progress, some beta stuff