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

closes #322

outputs/librato/README.md

@@ -0,0 +1,12 @@
+# Librato Output Plugin
+
+This plugin writes to the [Librato Metrics API](http://dev.librato.com/v1/metrics#metrics)
+and requires an `api_user` and `api_token` which can be obtained [here](https://metrics.librato.com/account/api_tokens)
+for the account.
+
+The `source_tag` option in the Configuration file is used to send contextual information from
+Point Tags to the API.
+
+If the point value being sent cannot be converted to a float64, the metric is skipped.
+
+Currently, the plugin does not send any associated Point Tags.