Update readme and composer.json

I've fixed up your readme and composer.json files so they reference your repository.

Can you also rename this repo to omnipay-adyen instead of adyen-omnipay (sorry, I got that round the wrong way).

Then once you have merged this pull request you can go to packagist.org and click "Submit package" and give it the github URL for this repository.

Once that is done add this to the list of available gateways for Omnipay so other people can use it :)