Replace the fragmented Swift client classes with one generic class

The object storage clients are separated into AccountClient, ContainerClient, ObjectClient and ObjectClientCustomizedHeader. There is no need for this, Swift has one of the simplest APIs in OS. They should be merged into one generic class that would also take care of resource cleanup. It should be easier to use and do all the return code checking, throwing an exception if commands fail. This way, it wouldn't be necessary to assert the response codes after each operation.