Create a Mock tracer implementation

[yurishkuro]

Most other language implementations support a Mock tracer. Having a mock tracer is useful for two reasons:

• it allows unit testing of the instrumentation with only the opentracing dependency
• implementing a mock tracer helps with ironing out the issues in the API itself, like Removes carriers. #38, as it serves as a reference implementation

[jcchavezs]

Great feedback @yurishkuro however there is already an issue for this: #21. Let's continue the discussion there.

[yurishkuro]

sorry, should've checked the issues