add log_logstash plugin to emit logstash json_event messages.
Merge branch 'feature-log-logstash' of github.com:pyr/collectd into feature-log-logstash

Conflicts:
	configure.ac
support emitting to stdout or stderr
bring in AM_CPPFLAGS
avoid possibility of muting log output
forgotten printf
small typos
document log_logstash
Merge pull request #360 from pyr/feature-log-logstash

add log_logstash plugin to emit logstash json_event messages.