Copy code and tests from signalfx-go

[pellared]

Copy code from https://github.com/signalfx/signalfx-go/tree/master/signalflow with changed import paths and updated dependencies.

The following will be done as separate PRs:

• Copy https://github.com/signalfx/signalfx-go/tree/master/signalflow/example most probably as a "test example"
• Add CI
• Add license headers
• Add releasing docs
• Add FOSSA scanning