[RFC] Merge service libraries into one repository and use git tree split

IMO it starts to be annoying that every payment has own repository & is not any way to get all of them in one time, also most of those implementation are really similar when you compare code bases...

What I'm suggesting is a merge into main repo all of those (could be hard to maintain at some point), or create new repository, i.e. `PayumGateway` (or something) and merge all those currently separated repositories (`AuthorizeNetAim`, `Be2Bill` etc.).

With this approch you could have in main catalog an `composer.json` file:
```json
{
    "name": "payum/gateways",
    "type": "library",
    "license": "MIT",
    "require": {
        "php": ">=5.3.2",
        "payum/payum": "0.7.x-dev",
    },
    "autoload": {
        "psr-0": { "Payum\\Gateway\\": "src/" }
    }
}
```
All gateways would be store i.e. `Payum\Gateway\Be2Bill`, and on that folder it will be same as actual content of repository with git tree split.

The approach is same as used by Symfony or Sylius.