Support Apache 2.4 configuration directives

Apache 2.4 uses a different server-side include syntax, but we can
direct it to use the old syntax via the SSILegacySyntax directive.

2.4 also uses different access control syntax: "Require all granted"
instead of "Allow from all", for example.

Accordingly, create a parallel examples/apache_24 directory that
contains sample Apache configuration files suitable for Apache 2.4 and
higher.

Also, amend the README to reflect Apache 2.4 and higher.

Signed-off-by: Dan Scott <dscott@laurentian.ca>
Signed-off-by: Ben Shum <bshum@biblio.org>