Use test helper.
Remove monkey patching of Banchmark module.
Refactor library and test suite.

- Refactor write method into three different methods with a single responsibility.
- Allow StatsD instrumentation to be removed.
- No more leaky tests, by removing instrumentation and proper stubbing
Simplify live tests by removing Thread
Merge pull request #31 from Shopify/refactor

Refactor module and test suite