Improved Installation Guide, Added Composer Support

Codeception makes writing tests pretty easy and painless, unfortunately getting it working with Yii Framework is pretty painful. This pull request attempts to alleviate much of those pains by making this dependency easier to install, and by providing more detailed instructions for getting Codeception working with Yii Framework. Here's what's included.

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### Composer Support
It doesn't make sense that Codeception provides support for Yii via composer, but this dependency has to be cloned into a project manually, so I added a single ```composer.json``` file so that this dependency can be installed via composer. The alias I used is ```codeception/YiiBridge```. 

Ideally, whoever owns this repository would setup a [packagist](https://packagist.org) repository for ```codeception/YiiBridge``` so that you wouldn't also have to add a ```requirements``` section to your ```composer.json``` file.

### More Detailed Installation Instructions
The installation instructions for this were pretty sparse, and left out a few important components, specifically with getting ```CodeceptionCHttpRequest``` working. I've updated the README to include more detailed instructions on how to get the Yii1 module working with codeception and actually being able to run tests with it.

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Let me know if there's anything you'd like to see in this PR.

Cheers!
Charles R. Portwood II