Add PHP module, php::ini resource, & remote debugging role

This change promotes 'mediawiki::php' to a separate, top-level 'php' module. It
also adds a 'php::ini' custom resource type, which manages /etc/php5/conf.d
conffiles (which extend php.ini). Finally, it adds a role , 'remote_debug',
which configures remote debugging via Xdebug. The role does not yet take care
of forwarding the port.

Change-Id: I99eaae3ba46c2f74423576fbf304cab43b39c739