Merge pull request #40 from JasonGross/HoTT-JMeq-polymorphic

Make JMeq universe polymorphic.
Fix eq/leq_constr_universes to use the enforce_eq/leq interface, dissalowing [Set = Prop] constraints right away.
Fix define_evar_as_product to add the proper constraints in case the evar is in Prop, implementing the correct product typing rule.
Merge branch 'trunk' of github.com:HoTT/coq into trunk