Remove code_statistics so it can be required as a gem

Whereas

* The code_statistics code in rails provides useful functionality
* It does not depend on rails in any way
* Its development does not need to depend on the rails release cycle

I have generated a rails plugin and have moved the code there, including tests,  so that it can be run as a separately installed gem.

I have renamed the gem [code_metrics](https://github.com/bf4/code_metrics) (rather than code_statistics) to prevent name collisions with the rails code.

The [extracted gem](https://github.com/bf4/code_metrics) is still a work in progress, but I think is complete enough to warrant a pull request to rails for consideration.  

I would rebase it to write a cleaner history when you are ready to move it into the rails project.

Thoughts?