[IronPython] Better dir() for Enum

[Sanghyeon Seo]

Let's illustrate this feature request with an interactive session:

>>> import System

>>> dir(System.IO.FileMode)
['Append', 'CompareTo', 'Create', 'CreateNew', 'Equals', 'Finalize',
'Format', 'GetHashCode', 'GetName', 'GetNames', 'GetType',
'GetTypeCode', 'GetUnderlyingType', 'GetValues', 'IsDefined',
'MemberwiseClone', 'Open', 'OpenOrCreate', 'Parse', 'ToBoolean',
'ToByte', 'ToChar', 'ToDateTime', 'ToDecimal', 'ToDouble', 'ToInt16',
'ToInt32', 'ToInt64', 'ToObject', 'ToSByte', 'ToSingle', 'ToString',
'ToType', 'ToUInt16', 'ToUInt32', 'ToUInt64', 'Truncate', '__class__',
'__init__', '__module__', '__repr__', 'value__']

>>> sorted(System.Enum.GetNames(System.IO.FileMode))
['Append', 'Create', 'CreateNew', 'Open', 'OpenOrCreate', 'Truncate']

I think there's no doubt that the later output is more useful. But I'm
not sure whether this feature request interferes with other possible
features of IronPython.

If the above is undesirable, at least this could be improved:

>>> print System.IO.FileMode.__doc__
no documentation available

Seo Sanghyeon