RTF writer: Added writeRTFWithEmbeddedImages.

* RTF writer:  Export writeRTFWithEmbeddedImages instead of
  rtfEmbedImage.
* Text.Pandoc: Use writeRTFWithEmbeddedImages for RTF.
* Moved code for embedding images in RTF out of pandoc.hs.
Set locale encoding in test suite.

This prevents a "commit buffer (invalid argument)" error on
Windows.