[Rd] Support writing UTF-8 output in Windows

[Sverre Stausland]

As recently discussed on Stack Overflow, R for Mac OS and Ubuntu (so
probably all Unix systems) can correctly write files with UTF-8
encoding, but R for Windows cannot:

http://stackoverflow.com/questions/19877676/write-utf-8-files-from-r

I strongly suggest that R for Windows should support this feature in
upcoming versions.

Sverre