Add NaN handling config options
input/collectd: Implement handling of NaN values
Add tests for the collectd NaN handeling
input/collectd: Better tagging on NaN

  * Don't overwrite existing tags
  * Allow the user to config the tag to add when a NaN is found
touch up statsd output
Fix percentiles on metrics filter
explain defaults
touch up
Change [A|a] by [Aa], etc.
Try more times to fetch from rubygems.org

Of the last 14 build failures, 6 of them were because of bad_record_mac.
Hopefully retrying more will work around this.
fix handling of more than 2 repetitions of same mail header
bluecloth is needed to build the docs
json_lines codec documentation is special since it isn't used with the file input.
Also fixed formatting issues with the config examples in the synopsis section.
shift specs selection from Makefile to leverage rspecs tags
fix timing issue and cleaups
tag :performance for exclusion
tag :redis for exclusion
tag :elasticsearch for exclusion
tag :broken for exclusion
tag :socket for exclusion