Added the mock_tracer to the opentracing export

[jhorowitz]

In reference to #89

[coveralls]

Coverage decreased (-26.8%) to 62.025% when pulling a76dc51 on jhorowitz:testing/mock into 1f32ae4 on opentracing:master.

[yurishkuro]

don't know why the build is failing.

is it possible to have a unit test that relies on the new import?

[jhorowitz]

I'm not sure what you mean. Do you mean write unit tests for mock_tracer or fool the coverage so the build works? I'm planning on writing unit tests for mock_tracer but I don't have enough bandwidth to do that today.

[yurishkuro]

I restarted the build, let's see if it succeeds. master branch is green, so no reason it shouldn't work in this PR.

What I meant by the import is if you look at #91 (comment) the import is done as import { MockTracer } from '../mock_tracer'; Instead is should be doing the import via ../index using the change from this PR.

[jhorowitz]

I see. I'll tackle this on Monday

[yurishkuro]

Done in #98