Make app-memcached more resilient

In preparation for http://postman.memetic.org/pipermail/observium/2013-August/003605.html

Currently the memcached agent sends all data is has as one value per line and the parser assumes a certain list of values. Depending on the memcached version the agent might send different data than the poller expects. Because the poller only receives a lit of values it cannot detect this.

This patch changes the data format from values-only to key:value pairs. That way the poller can always find the correct values, even if the data provided by memcached changes in the future.

The poller still can parse the old format for agents that haven't been upgraded yet. It does check if the number of lines received matches the number of values expected. If there is a mismatch it will ignore the memcached instance instead of mis-parsing the data as it currently does.