Fix links between modules in HTML docs output

The relative path to the root directory was wrong.

Probably the code handling the ActiveState distribution case also needs
the same fix, but I can't test it and hence haven't included it in this
fix.

Signed-off-by: Michael Wild <themiwi@users.sourceforge.net>
Merge pull request #15 from themiwi/patches/fix-html-links

Fix links between modules in HTML docs output
Fix #72176 ([PATCH] pod2html will fail with an empty podpath in some cases)
Fix htmlroot path

It could sometimes end up being empty, causing Pod::Html to complain and
tests to fail
Add --pureperl-only support
Prepare for release 0.4005