Decouple JUnit

[raniejade]

Currently we are tightly coupled with JUnit as we depend on its runner. This has its advantages like getting out-of-the-box tooling support (IDEs, build tools, etc ...). However in the long run it blocks us from doing amazing things, like having first-class tooling support and proper reports (#5).

Tasks

• Implement SpecRunner - this will run a single spec, we also might need to implementing something like JUnit's RunNotifier.
• Rework JunitSpecRunner to use SpecRunner