add librato output plugin, update datadog plugin

[jipperinbham]

The following PR serves 2 purposes:

• add a new output plugin for sending metrics to Librato
• update datadog output plugin to skip any metric where the value cannot be converted to a float64

Also added READMEs for each output.

[sparrc]

looks great, thanks much @jipperinbham 👍

[otoolep]

Go style tends to capitalize 3-letter acronyms. So this should be APIUser.

[jipperinbham]

good to know, I assume I'd need to annotate it with toml:"api_user" as well?

[otoolep]

apiUser would be more inline with what we would do in core.

[jipperinbham]

ok, my understanding was fields needed to be exported in order to be set properly from the toml config?

[otoolep]

I'm talking about the JSON encoding tag names. They can be lower case.

So be clear, the core team would write:

type Librato struct {
    APIUser string `json:"apiUser"`
}

[otoolep]

Some style suggestions, which would be more inline with how we code in Core. Not a big deal however.