Add composer support

This patch adds initial support for composer. Hopefully I got this right.

In order for package consumers to have this work out of the box, ie without additionally [adding the repository URL to the local composer configuration](https://getcomposer.org/doc/05-repositories.md#vcs), the repository still needs to be registered with https://packagist.org. I guess anyone can do that, not sure, I never did it before.

Once merged and registered it should be possible to include the plugin using for example

```json
{
    "require": {
        "bmcclure/cakephp-media-plugin": "2.*@dev"
    }
}
```