Add missing enum attributes

[JelleZijlstra]

Fixes #854 (unless there's more I'm missing).

Tested with this file:

import enum

class A(enum.Enum):
    b = 1
    c = 2

reveal_type(reversed(A))
reveal_type(A.b in A)
reveal_type(A['a'])
reveal_type(A.__members__)

With mypy master this produces:

../bin/enum36.py:7: error: Revealed type is 'typing.Iterator[enum.Enum*]'
../bin/enum36.py:8: error: Revealed type is 'builtins.bool'
../bin/enum36.py:9: error: Revealed type is 'enum36.A'
../bin/enum36.py:10: error: Revealed type is 'typing.Mapping[builtins.str, enum36.A*]'
../bin/enum36.py:10: error: Invalid method type

The first line should be Iterator[A] but that's due to python/mypy#3210, and the last error is wrong but that's due to python/mypy#3223.