amqp plugin: add support for Graphite output

amqp plugin: add support for Graphite output

This commit implements "Graphite format" for AMQP Plugin.
The AMQP plugin will be able to directly output a valid
Graphite metric format (<metric name> <value> <timestamp>\n).
This is very useful when the Graphite server is directly reading
from an AMQP broker. You can then avoid having a proxy somewhere
doing the conversion between PUTVAL or JSON metrics into Graphite
format.

Signed-off-by: Florian Forster <octo@collectd.org>