Allow to specify list of IPs in a body on maintenance.flag which will be granted access even if the flag is on

This is a new functionality allowing to specify list of IPs which will be granted access to the website even if maintenance flag is on.

The IPs just has to be specified in the `app/etc/allowed_ips.xml` file. The format of the file has to be the following:

    <?xml version="1.0"?>
    <ips>
        <ip>33.33.33.1</ip>
        <ip>178.178.189.9</ip>
    </ips>

The names of nodes doesn't matter. The just has to be one root node and then each IP has to be stored in a child node with the same tag.

This functionality can be useful for release processes when the developer needs to make sure that the website is running as it should while general public is not able to access the website. Another use case can be the occasion when the problem critical is found on production website and seems to be a server specific. In this case developer will be able to put the maintenance flag but still access the website by himself in order to trace the problem.

The implemented feature is not covered with tests as "maintenance.flag" related functionality is not covered in stock.

There is no XML validation implemented. The `allowed_ips.xml` is supposed to be a valid XML file. If file doesn't exits the script will just move on considering there are no allowed IPs.