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 

I just want to make sure that you want to expose @todo in the public docs.


![2014-07-09_11-42-37](https://cloud.githubusercontent.com/assets/466104/3522498/abbec4e6-074d-11e4-99da-0f8d139534a8.png)