refactor(frontends/lean): replace collect_metavars with metavar_closure helper class
feat(library/io_state): add default constructor to io_state
feat(library/unifier): keep normalization result when processing universe level unification constraints
refactor(library/unifier): add option m_discard too unifier, if m_discard == false, then unsolved flex-flex constraints are returned, the unifier also does not apply "last resource" techniques that may miss many solutions.
feat(frontends/lean/placeholder_elaborator): perform class-instance resolution in a completely independent unifier object, it also triggers the resolution when expected type does not contain metavariables, closes #175, closes #173, closes #68
chore(library/data/quotient): rename aux.lean to util.lean (fix problem with cloning lean repository on Windows machine)

See
http://stackoverflow.com/questions/3689137/error-git-checkout-index-unable-to-create-file
for additional details
test(tests/lean/run): add section test
fix(frontends/lean): name clash inside section, fixes #181
feat(library/data/category.lean) the definition of category doesn't depend on 'mor' anymore; make iso a class; add theorems
fix(library/data/category): minor problem that was being masked by bug #182, fixes #183