This patch is to fix bug 1693753. Basically, os.rename(src, dst) fails on windows if dst already exists. This is because MoveFile() is used to implement rename. This patch replaces this API with MoveFileEx() and uses the flag MOVEFILE_REPLACE_EXISTING causing the API to replace the dst if it already exists. This brings the behaviour in line with unix. Note that, I also use the flag MOVEFILE_COPY_ALLOWED which is required if dst is on a different volume. However, moving to a different volume may not be atomic operation (I am not sure about this) as the msdn doc says that move in this case is simulated with CopyFile() and DeleteFile(). 

The patch also includes a test case and doc update. Please let me know if the location of test case and name of test function are ok. Also, MoveFileEx() is only available on NT+ windows but it should be ok as support for Win9x, WinME is removed in 2.6.