environment variables need docs

## Motivation

The environment variables need documentation.

I tried documenting with yard, but the results were terrible.

I decided to use a Markdown file (ENVIRONMENT_VARIABLES.md) and tell yard to include it in the documentation.

The markdown itself does not look good when viewed on its own.  However, once it has been parsed by yard, it looks great (IMHO).

## Todo

- [x] @krukow Should we include the Keyboard Variables?
- [x] @krukow Should we include the Predefined Steps Variables?
- [x] @krukow Should we include the Gem Developer Variables?

This is my first pass at the docs.  I think I got most of it right.

## Preview

The best way to see the changes is to build the docs locally with yard (see README_YARD.md).

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![2014-08-14_14-28-11](https://cloud.githubusercontent.com/assets/466104/3920039/b93a1e60-23b1-11e4-9184-ee243be4a5d6.png)

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![2014-08-14_14-40-35](https://cloud.githubusercontent.com/assets/466104/3920044/c6862a3c-23b1-11e4-9fbf-2a71ff090d0d.png)

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![2014-08-14_14-41-15](https://cloud.githubusercontent.com/assets/466104/3920046/cc97cd7c-23b1-11e4-9c0a-1c869b5e7db6.png)

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