Production rule: docbook -> epub

A production rule would be desirable that given a single docbook file produces an epub package. The styles are present in docbook-xsl (although without profiling). The difficulty comes from the fact that two directories are created, the images need to be copied in the proper location as an additional step, a few additional passes are required and the zip created.

The rule would receive a single file name and perform all these steps. The trickiest one could be the inclusion of the right images or auxiliary files, but parsing the opf file could help detect anomalies.

This issue is likely to be open for quite some time.