write_graphite plugin: Adding TCP/UDP support via config

UDP allows to "fire-and-forget" data while not bringing the servers down
due to TCP's connection oriented overhead.

The new versions of Graphite support both TCP and UDP streams so it made
sense to add UDP support.
write_graphite plugin: Adding UDP support to docs, README, etc.
write_graphite plugin: Adding LogSendErrors boolean flag

Sometimes we don't want to hog the syslog with errors that are produced
when the graphite server is down.

Especially for UDP, we want to be able to truly fire-and-forget
(remember to monitor UDP packet receipt failures though :) and having
errors written to the syslog is a burden that should be avoided.
write_graphite plugin: Adding LogSendErrors to docs, README, etc.
write_graphite plugin: Adding myself to AUTHORS :)
write_graphite plugin: config should fail on error

This is the preferred way of handling config, as it can be seen in snmp.c. It's
important to fail on a config error to simplify code in other callbacks.
write_graphite plugin: avoid flooding the log with the protocol error check

Moving the protocol error checking from `wg_callback_init` to `wg_config_node`.
This avoids flooding the log, as suggested by Florian Forster (@octo).
Merge branch 'feature-write_graphite-udp' of git://github.com/jjmaestro/collectd