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
Added option to disable certificate validation

If a IMAP-server is using a self-signed certificate, this patch adds a "verify_cert" option to the module that is by default set to true (old behavior), but if set to false it will disable the verification check. The result is that IMAP TLS with self-signed certificate will not crash logstash.
Added option to disable certificate validation
New collectd codec to go with the new and improved multithreaded udp input plugin.  This makes the collectd input plugin obsolete.

# The Logstash input configuration can be as simple as:

```
input {
  udp {
    port => 28526
    buffer_size => 1452
    codec => collectd { }
  }
}
```
Figured out why `@types` wasn't initializing before register.  It needed to be above `super` (thanks @jordansissel)
Add tests for the collectd codec
Merge pull request #1 from pieterlexis/collectd-untergeek

Add tests for the collectd codec
Added warning when registering insecure state
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.