Address file system path compatibility issues

Closes dompdf#1191

Use strict equality when testing for 0

Restrict access to sensitive www content to authenticated users

Protocol relative url support for build url helper

Closes dompdf#1026

tweak instructions for non-composer installations

Tagging release 0.6.1

Tagging 0.6.0 release

Tagging version 0.5.2 release (dompdf#695)

Tagging version 0.6.0 beta 3 release (dompdf#695)