Add support for custom site directory names

The path to the site directory is determined by the
HOTMELT_SITE_DIRECTORY constant. For standard setups this constant is
automatically defined in the default dispatch.php. To use a custom site
directory name, create a custom dispatch.php in your site directory and
define the HOTMELT_SITE_DIRECTORY constant before requiring the default
dispatch.php

Remember to update your .htaccess, so the rewrite rules point to your
custom dispatch.php.
Merge branch 'feature/custom-site-dir' into develop

The path to the site directory is determined by the
HOTMELT_SITE_DIRECTORY constant. For standard setups this constant is
automatically defined in the default dispatch.php. To use a custom site
directory name, create a custom dispatch.php in your site directory:

``` php
<?php
define('HOTMELT_SITE_DIRECTORY', dirname(__FILE__));
require_once(dirname(__FILE__).'/../HotMelt/dispatch.php');
```

Remember to update your .htaccess, so the rewrite rules point to your
custom dispatch.php.

closes #4