write up docs and visibility modifiers on the Core module

This adds documentation for all the methods in `Calabash::Cucumber::Core`.

- [ ] @jmoody - please review 


@krukow 

Should this have a @return [Array] tag?

Do we want to go through the trouble of declaring what can be in the array?

I think this is the list:

```
[Array <Hash>]
[Array <String>]
[Array <Number>]
```

If you want, I can add changes to this PR.