Add mod_rewrite support for the API

The API is implemented for HTTP requests via the api/http.php module.  Where
supported (by Apache), support rewriting URIs from something like
http://domain.tld/api/tickets.xml to
http://domain.tld/api/http.php/tickets.xml, so the explicit usage of the
http.php script is not required.

api/.htaccess

+Options +FollowSymLinks
+RewriteEngine On
+
+RewriteBase /api/
+
+RewriteCond %{REQUEST_FILENAME} !-f
+RewriteCond %{REQUEST_FILENAME} !-d
+
+RewriteRule ^(.*)$ http.php/$1 [L]