code refactoring, alias and directlink request, composer phpunit

Refactoring of existing classes to be able to use three kind of different requests: EcommercePaymentRequest, CreateAliasRequest and DirectlinkPaymentRequest.

Added phpunit dependency for development.

Edited / added unittests to use added classes.
Response requests for Ecommerce, DirectLink and CreateAlias + unit tests + updated readme
added (some failing) tests for Alias VO
cleanup code
remove deprecated use of git submodules (in favor of composer)
Cleanup after pull request remarks
Ogone unit tests using Guzzle
Minor changes after feedback
Edited FormGeneratorTest to use uppercase
Changed unit tests to use constants
rename FormGeneratorTest.php to be consistent with PSR-0 autoloading
PaymentRequest and PaymentResponse interfaces added for constants
UrlGenerator deps updated to EcommercePaymentRequest only
refactoring SubscriptionRequest linking to EcommercePaymentRequest