Add composer support

This allows the plugin to be installed with composer using [composer installers](https://github.com/composer/installers). This ensures it gets loaded correctly into the `Plugin/` directory, and named according to the `installer-name` attribute. In this case `Mongodb`.

I've also added two branch aliases, as I can't seem to get versions to work with your current release tags.

I used the `composer.json` found in [CakePHP's Debug Kit](https://packagist.org/packages/cakephp/debug_kit) as a basis. It would also be wonderful if you could submit the package to the composer library for even easier installation.

Thanks for all your hard work and an excellent plugin.