Adding callbacks to EnumField and lots of EnumField tests

  - this feature allows conversion of large ranges where
    all entries coalesce to the same representation without bloating
    a dict. For instance, 0x0 to 0xffff all display as "toto" except
    a few odds 0x2 which are displayed as "tutu".
    Before this feature, you needed to write
    enum = {x: 'toto' for x in xrange(0, 0xffff)
    enum[2] = 'tutu'
    XShortEnumField('test', 0, enum)