Add gRPC-OpenTracing interceptors for Go

[bhs]

See the README.md for usage examples, etc.

This is possible thanks to https://github.com/grpc/grpc-go/pull/867.

[bhs]

cc: @dominikh per our conversation on Gitter about this

I would like to include more information about the peer (via tags/etc), but unfortunately I'm not sure where to access that given what's (not) exported in gRPC.

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[bhs]

Per the google CLA thing: "I did it"

[googlebot]

CLAs look good, thanks!

[bg451]

Everything looks good to me, although I would recommend adding an Option that allows users to specify a set of tags that'll be applied whenever a new span is created in the interceptor.

[jmacd]

👍

[bhs]

Comments addressed... merging.