Externalization of the docbook xsl dir

I recently work on 2 projects in parallel and the needs of the documentation output pdf were differents.
I had to switch to comment and uncomment some parts of common.xsl file... and I decided to create 2 distincts directories...
So in this pull request, I update the fopdf file in order to take parameters :

. -x parameter  for specifying the directory where your own docbook-xsl directory is
. -t parameter for type (pdf,ps or fo)

NOTE: I did not do the work for the fopdf.bat... no Windows at home...

I update README.adoc to explain this.