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