allowing dynamic loading of memcached backend class

Hello!

To this day, this module relied on Cache::Memcached to interact
with memcached servers. Although there is nothing wrong with that,
Cache::Memcached is pure-perl and thus can be slower than C/XS
implementations like Cache::Memcached::Fast::Safe and
Cache::Memcached::libmemcached, which provide very similar interfaces.

This patch allows the developer to choose from its preferred memcached
backend class via the 'memcached_backend' configuration file, and still
defaults to Cache::Memcached so it doesn't break any apps who depend
on this plugin.