Adding examples on enumerations
Started adding support for enums
Adding support for enumerations

  - Introducing new type TEnum

  - If

    enum E { A,B,C }

    then

      E       :: enum E

      E.A     :: { number | v = 0   }

      E[0]    :: { string | v = "A" }

  - In unification / subtyping: enum E behaves like an object literal
Cleaning up
Merge pull request #57 from UCSD-PL/enumerations

Enumerations
Merge branch 'master' into arbitrary_tags
Fix in lookupAmbientType
Adding keyVal refinement in getProp.

  - Fixes in Sub - unification
Fixes in unification, zipTypes