Add Apache 2.4 compatibility

As of Apache 2.3 the `Order`, `Allow`, `Deny` directives are replaced by the `Require` directive, making Apache 2.2 configuration not compatible with Apache 2.4.
This patch checks for [`mod_authz_core`](http://httpd.apache.org/docs/2.4/mod/mod_authz_core.html) base module presence (available from 2.3 release) to enable the correct syntax.

Closes issue #5.