Internal links failed

Attached two patches for project2fo and fo-body that make internal links work. It might be considered a bit hacky and probably needs some more testing but it works fine for me and I am not going to spend more time on it so I make it available. Here's what I've done:

• so far internal links used the generate-id() function which does not work for relative links because the id is unique for the current xml node only. I have replaced that by an algorithm that constructs the id from the path of the current document and the href tag. As such, links should work in complete analogy to the xdoc plugin. In particular, id's are also generated for sub/section names (replacing spaces by underscores, again like xdoc).

• the patch for fo-body also contains a fix for MPPDF-13: the location of image sources is constructed relative to the location of the current document.

• the patch for projet2fo also contains a fix for MPPDF-36 with the solution I indicated there, ie, the footer is simply ignored. It might be included somehow in the future, but this at least avoids a complete failure of the pdf generation.

• a minor fix: in fo-body, the baseline-shift tag in the "sup" template (superscripts) was set to "sup", it should be "super". However, sub- and superscripts still don't work even though they are marked as implemented by the fop project (http://xml.apache.org/fop/compliance.html). This is probably a fop issue so I didn't want to open a JIRA issue for that.

Finally MPPDF-23 also seems to be fixed, even though this was the case already before applying my patches.

Thanks a lot Lukas.
This is a very good patch.
I have actually applied it locally but I have a problem generating the maven's doc.
I must fix a duplicated id error before to commit it.

I just checked myself with the maven docs because I had no problems with any of my projects. Indeed, for maven the pdf generation fails with a duplicate id. This is because the href="/reference/maven-jelly-tags/index.html" tag appears twice in navigation-pdf.xml. This is related to MPPDF-26 which has to be fixed on a more general level. For now, you will have to remove one of the two href's in navigation-pdf.

patch applied. Thanks to Lukas Theussl.