Refactoring: Abstracting the BDD parts out to a shared package

In order to support a number of new features, I will need to represent the core BDD principals in code.

I do not want to pollute the root `mspec` package. Instead, I would prefer to abstract, create and move the core BDD parts out to a dedicated package. Most likely:

```
github.com/eduncan911/mspec/bdd
```

This could have some nice side effects, besides supporting other code and runtimes I plan on building. For example, someone could import that package directly and use it to create their own test runner, web-driven BDD framework, etc.

See #4 for details.