RTF to DTBook script

[bertfrees]

This script exists in Pipeline 1. It's based on some Python script to convert the RTF to XML, and then there is an XSLT to convert that to DTBook.

[josteinaj]

As with #129, I think it would make sense to rather have a RTF to EPUB 3 script, and a EPUB 3 to DTBook script. Following the method outlined in #101 (comment).

Edit: I created an issue for it here: #137