Added constant to retrieve Windows' Documents path

Created by: francogarcia

Currently, the function OS.get_system_dir(OS.SYSTEM_DIR_DOCUMENTS) returns an empty string in Windows.

From the commit message:

According to MSDN's documentation ([(https://msdn.microsoft.com/en-us/library/windows/desktop/bb762181(v=vs.85).aspx)]), CSIDL_PERSONAL represents the default path to Windows Document's folder:

"Previous to Version 6.0. The file system directory used to physically store a user's common repository of documents. A typical path is C:\Documents and Settings\username\My Documents. This should be distinguished from the virtual My Documents folder in the namespace. To access that virtual folder, use SHGetFolderLocation, which returns the ITEMIDLIST for the virtual location, or refer to the technique described in Managing the File System."

SHGetFolderPath() may fail in some conditions for the Documents folder, though. I believe failures may occur when the user moves the documents folder elsewhere, or uses a symbolic link.

Thanks. Best regards.