[#27111] Maintenance page breaks installation process.

Problem description from https://redmine.previousnext.com.au/issues/27111 : 
> According to http://drupal.stackexchange.com/questions/94010/custom-maintenance-page-breaks-module-install-process :
>
>> Apparently, Drupal limits/locks access to the database during module/theme install, thus triggering the maintenance-page--offline.tpl.php
>
> Since the current maintenance-page--offline template does not include a $content variable, going to install.php will only show the offline message and not the installation form.

Screenshot of initial /install.php page before this PR:
![screen shot 2014-11-10 at 4 55 31 pm](https://cloud.githubusercontent.com/assets/33429/4973230/fed99338-68b7-11e4-9581-2d2d6cc2fbea.png)

Screenshot of initial /install.php page after this PR:
![screen shot 2014-11-10 at 4 54 48 pm](https://cloud.githubusercontent.com/assets/33429/4973234/0e1d41b4-68b8-11e4-911a-ff72d493a7c2.png)