configure: Show default values in env help strings
configure: Add an enviornment variable for WWW_ROOT
configure: Add WWW_PREFIX variable

Issue #151.
htaccess: Define a document prefix

Issue #151.
_page.html: Remove links variable

Issue #151.
config.php: Add a Pip_Links class
template.php: Add the Pip_Links::val() to content

Issue #151.
html: Prefix links with {{ _links.root }}

Issue #151.
s.php: Prefix links with $_SERVER['DOCUMENT_PREFIX'];

Issue #151.
_navbar: Update reference to setup

Issue #151.
htaccess: Prepend WWW_PREFIX to ErrorDocument

Issue #151.
htaccess: Only apply PHP redirect if file exists

Issue #153.