Fixes first part of issue #83: relative include not processed correctly

The problem is with the basedir property defaults sent to asciidoctor.
When not specified explicitly, the baseDir is set to projectDir,
which makes asciidoctor fail to find the relative file.

This patch changes the default to be the dir of the processed file.
Fixes second part of issue #83

The problem here is actually not with includes per se but
with Asciidoctor's handling of absolute paths in Windows.
They are apparently treated as URIs and rendered as links only, unless
allow-uri-read attribute was specified.

This patch works around this problem by setting projectdir and rootdir
attributes as relative paths instead of absolute paths.
Merge pull request #85 from afolmert/master

Issue #83 fixes