added original-base and ref options to fileset-add-entry and created fileset-add-ref

test doc:

<?xml version="1.0" encoding="UTF-8"?>
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" xmlns:c="http://www.w3.org/ns/xproc-step" version="1.0" xmlns:px="http://www.daisy.org/ns/pipeline/xproc">

    <p:output port="result"/>

    <p:import href="http://www.daisy.org/pipeline/modules/fileset-utils/xproc/fileset-library.xpl"/>

    <px:fileset-create base="file:///tmp//dir/"/>
    <px:fileset-add-entry href="fileC.xml"/>
    <px:fileset-add-entry href="..///fileA.xml" ref="./fileB.xml" original-href="./../fileA.html"/>

</p:declare-step>
base-uri(.) in xslt, warning for missing file, fixed comment
Merge pull request #2 from daisy-consortium/fileset-file-refs

added original-base and ref options to fileset-add-entry and created fileset-add-ref