Add SignalFx output plugin

[ewbankkit]

Required for all PRs:

• CHANGELOG.md updated (we recommend not updating this until the PR has been approved by a maintainer)
• Sign CLA (if not already signed)
• README.md updated (if adding a new plugin)

For those whose love Telegraf for metrics collection but prefer to use SignalFx for storage and analysis.
May need to revisit SignalFx datapoint type once/if #1694 is merged.

[ewbankkit]

OK, need to add back opencontainers to Godeps...

[ewbankkit]

• Rebased with latest master
• Added support for Telegraf metric types
• Added name sanitization, similar to Prometheus output plugin

[ewbankkit]

I'll follow and integrate with the outcome of #1717 and #1718.

[nhaugo]

@ewbankkit Sorry about the delay on the merge, but it looks like you figured out the reason. We should be able to get this merged once we have the plugins checked in.

[ewbankkit]

Is an exec output plugin in the works: #544?

[sparrc]

@ewbankkit not at the moment, but we may want to prioritize that for a 1.x release to better enable external plugins.

[ewbankkit]

Addresses #923.

[ewbankkit]

Closing this PR as an exec output plugin seems to be the longer-term way forward.
See #1999.

[amoghe]

@ewbankkit any chance we could revive this and have a built-in signalfx output plugin (seeing as how the exec one never fully landed)?

If you could point out the remaining work you'd like to see as part of this PR, I'd be happy to take over and see it through to completion.

[ewbankkit]

@amoghe Please feel free to take this over.