Update cache code to INI style configuration

Change the defines to config_get and add one cache option and one option
to define memcache_servers. Mention the required dependency to get
memcached working in the INSTALL file.

Signed-off-by: Jelle van der Waa <jelle@vdwaa.nl>
Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>