Fix issue 379 - regression with page links

See https://code.google.com/p/daisy-pipeline/issues/detail?id=379

Relative links were not correctly computed when the base URI passed as
parameter was that of a document and not a directory.
Rather than forcing caller to always pass a URI of a directory, the XSLT
now systematically computes the URI of the directory from the URI passed
as the `base-uri` paramater.
Fix another issue with page list URIs relativization

The relativization was looking at the doc's base URI with the XPath
`base-uri(/)`. Due to XProc current limitations it's hard to
set a doc base URI and it's always safer to look at the root element's
base URI, this change hence uses `base-uri(/*)` instead.
Merge pull request #33 from daisy-consortium/issue/379

Fix issue 379 - regression with page links