[registry] add PaymentRegistryInterface::getPayments method.
[registry] add SimpleRegistry::registerStorageExtensions helper method.
[doc] update doc to use simple array model and registry helper.
[doc] add installation part to get it started.
[doc] add your-order-integration section.
fic test compatibility with 5.3
Merge pull request #75 from Payum/registry-simply-registring-storage-extensions

Registry simply registring storage extensions