Add better BigEndian support via BitConverter

The following suggestion was made by Keith Hill, via the BCL Web Site:
"We have to deal with binary files that are in a standard format which is big endian. It would have been nice to use the BitConverter and/or BinaryReader/Writer classes but they are fixed to support only little endian. Could you consider updating these classes to support big endian byte order?"
Keith, this has not been the first time we've heard this kind of request, and we are already tracking it for consideration in a future version of the framework. It's great to get another vote for the feature, so it gives it more weight, thanks!