Add composer.json

Adds a composer.json file so that this file can be included via a projects's own composer.json file, e.g.

```javascript
{
    "name" : "example/app",
    "description" : "ExampleApp",
    "license" : "BSD",
    "repositories": [
        {
            "type": "git",
            "url": "https://github.com/kalaspuff/redis-cache-zend-framework.git"
        }
    ],
    "require": {
        "kalaspuff/redis-cache-zend-framework": "dev-master"
    }
}
```