Shape up the code for the bundle jar allowing collector selection

[carlosalberto]

After #235 was merged a few items are left to be done for lightstep-jre-tracer-bundle:

• [ ] Add a dependency to collector-grpc in lightstep-jre-tracer-bundle and make sure it works just fine.
• [ ] Shade related/important libraries that are brought as part of collector-grpc, similar to how we do that for the shadow artifact (many more libraries might need to be shaded).
• [ ] Consider shading okhttp libraries as well.

The idea is that users of the bundle jar do not get version conflicts when using grpc and related components (and okhttp to a minor degree).

[carlosalberto]

@malafeev Think you can take this one also? This one will require some digging and verifications here and there, but it's something we really need to have ;)

[malafeev]

sure!

[andrewhsu]

Looks like PR #244 is ready.

Note: also need to look into shading google protobuf later.