initial version of documentation, xspec testing

 * initial version of documentation for HTML/DTBook mapping
 * only convert pagebreaks when it's a span or div (epub3-to-dtbook)
 * removed global variables to support xspec
 * enabled xspec testing in POM (currently requires unpublished version of xspec-maven-plugin though)
first attempt at creating a single mapping document

both the dtbook-to-epub3 and epub3-to-dtbook conversion paths will
be described by this single mapping document.

A fourth column containing the dtbook element names has been added
but the descriptions (i.e. what classes must have what values etc
for the template to match) has not been fixed and all the descriptions
in the table cells must be updated.
fixed descriptions for the DTBook elements
finished the human-readable html/dtbook mapping
deleted dtbook-to-epub3.html (outdated file)
Merge pull request #88 from nlbdev/documentation

Documentation and XSpec testing