Issue #27: Use more accurate paths to avoid depth issues.

The configuration reference uses some additional path depths, for
instance:

    config-reference/compute/section_compute-configure-migrations.xml

These files will in turn include content from ../../common. As the
regular expressions used to re-map references to common content were
overly greedy such a path was incorrectly rewritten as common rather
than ../common.

This update to the XSL should ensure that only one level of depth is
removed from such references. This does not apply to image paths which
Publican always evaluates from the top level of the tree, rather than
relative to the XML file including them. Additional work is also
anticipated for dealing with samples.

Additionally it was previously possible that logic for replacing common
names like samples, figures, and common would pick up instances where
these were in fact substrings rather than the full directory names. To
avoid this path match and replace operations now include the trailing slash.

Handling of samples has also been improved to better ensure that they
are found regardless of where they are included from though it is still
likely even with these changes some cases will not work.

Further refactoring of this XSL now that it works with the most complex
book (configuration reference guide) is most definitely required.