- changes to the Debian package for compatibility with Apache 2.4

The current Debian package configuration of mod_tile is geared towards Apache-2.2, which is present in Debian Lennie and Ubuntu 12.04.

However, the more recent releases Debian 'Jessie' and Ubuntu 14.04 use Apache 2.4 packages, which are incompatible with the current mod_tile package configuration for the following reasons:
- There are no "prefork" or "threaded" packages of Apache anymore to select a worker process modell. Thus, mod_tile cannot/should not depend on these packages. For the same reason, there is also no apache2.x-common package anymore.
- Apache site configuration files now have the file extension ".conf" (instead of no extension at all). The Apache tools "a2ensite" and "a2dissite" used in the mod_tile package configuration scripts require that extension to be present.
- the default Apache site is now called "000-default" instead of just "default"

This patch fixes those incompatibilities.