Allow to define path to look for standards

This PR allow to use a different path to look for standards than the PHP_CodeSniffer one.

In example, in a coke composer installation, with this composer.json : 
```json
    "require-dev": {
        "atoum/atoum-bundle"                  : "dev-master",
        "m6web/coke"                          : "~1.0",
        "instaclick/symfony2-coding-standard" : "dev-master"
    }
```

And this .coke file : 
```shell
# Standard used by PHP CodeSniffer (required)
standard=Symfony2
standard-path="vendor/instaclick/symfony2-coding-standard"
```

To do code validation only with in-project dependencies