patch to add support for rich header

[ange]

I know it's not standard and variable, thus would not fit into the
strict structure format.
I also considered keeping it in peutils, but I prefer to have it
loaded already when fast_load is false.

[Ero Carrera]

Given that should be a rather quick computation I would rather add it
to pefile with some checking in place to make sure there's a Rich
header to parse.

[ange]

I attached the patch as pefile_rich_header.diff