Develop

Adding a few things to the gem:
1.  Lightweight integration tests.
2.  Logging.
3.  Tests are no longer dependent on a live zipkin collector being available.

@AimeeRose Please review and merge when satisfied.

All specs pass:
```
Finished in 2.27 seconds
56 examples, 0 failures
Coverage report generated for RSpec to /Users/mszenher/Documents/work/zipkin-tracer/coverage. 405 / 430 LOC (94.19%) covered.
```