Feature/html5 maintenance page

I just made a simple HTML5 page for maintenance.

Usage:

Rename to index.php and it will know that the site is undergoing an upgrade.

Rename to unavailable.php and it will know that the site is up, so the only files that are 'down' are certain development files.

Rename to something else, and you get a useless message. **Don't change the file type! XD**

The file is self contained, so no external files connected except for Google Fonts' Lato tff file.