add librato output plugin, update datadog plugin to skip non-number metrics

closes #322

outputs/datadog/README.md

@@ -0,0 +1,9 @@
+# Datadog Output Plugin
+
+This plugin writes to the [Datadog Metrics API](http://docs.datadoghq.com/api/#metrics)
+and requires an `apikey` which can be obtained [here](https://app.datadoghq.com/account/settings#api)
+for the account.
+
+If the point value being sent cannot be converted to a float64, the metric is skipped.
+
+Metrics are grouped by converting any `_` characters to `.` in the Point Name.