Added variables in composer.json

Variables can be referenced in the composer.json.

An example:
```json
{
    "require": {
        "symfony/console": "%symfony.version%",
        "symfony/process": "%symfony.version%",
        "symfony/yaml": "%symfony.version%",
        "symfony/filesystem": "%symfony.version%"
    },
    "variables": {
        "symfony.version": "v2.3.1"
    }
}
```