Add GrpcSender for jaeger-collector

[Falco20019]

Which problem is this PR solving?

• Fixes Support for GrpcSender #139

Short description of the changes

• The jaeger-collector got an GRPC endpoint. This can be used instead of Thrift. This add a separate package Jaeger.Grpc (like Jaeger.Thrift).

As discussed in #116, we should invert the dependecy before releasing 0.4.0 so that the user can choose between Thrift or Grpc and does not have to get all the dependencies (or non in case they implement ISender themselves). But this is outside the scope of this PR.

[Falco20019]

@yurishkuro Looking at the collector.proto, I assume we will have a agent.proto in phase 3 of jaegertracing/jaeger#773. My proposal is, to implement the jager-collector GRPC endpoint like we have with the HTTP endpoint for users skipping the jaeger-agent.

I would also propose to add a new environment flag JAEGER_GRPC_MODE to define if contacting jaeger-collector or jaeger-agent. As we only have collector mode right now, I would also spare it for now and just keep it as concept.

[yurishkuro]

Would be good to extend crossdock tests with this mode

[Falco20019]

This needs to be rebased once #151 was merged.

[Falco20019]

As #151 should be merged today or tomorrow, I will recreate this PR in a follow-up since it's easier as rebasing this.