Add support for IncludeOptional directive added in httpd 2.3.6.

This seems to be used by default on newer systems (at least Fedora 20
has it) causing the apache plugin to not discover large parts of
httpd inventory subtree.
Merged pull request #58 from metlos/feature/support-IncludeOptional-in-apache-plugin

Add support for IncludeOptional directive added in httpd 2.3.6.