Rename examples to testbed.

[carlosalberto]

Renamed examples to testbed, along the namespace name, to align to opentracing/opentracing-java#185

[MikeGoldsmith]

Was this really necessary? I understand the need to get the core API to match the Java implementation as close as possible but I think this is superfluous, with Examples being a more accurate description of what is there.

[carlosalberto]

@MikeGoldsmith It was mentioned in the past (in the Java repo) that the examples name was confusing as some of the scenarios are kinda exotic, and also because the purpose of these so-called examples was more to test out the API (along with changes and experimental features), more than being actual examples (and thus, confusing for newcomers).

Other repos do have actual examples, so we may want to add a few of them ;)

[cwe1ss]

Hmm. We had to change the examples/testbed code anyway because of async/await so maybe our code is closer to being actual "examples" than in Java?!

I'm ok with both so feel free to merge/not merge as you like!

[cwe1ss]

@carlosalberto @MikeGoldsmith how do you want to proceed with this?