Added class CloseFile.
Added integer codec functions.
create_header uses int_codec function
Added ProtObjFactory that implements the ReceivableFactory interface.
Use encode_32bit_int instead of having the implementation itself.
Made FileHandle a Receivable

act is not defined yet, though.
Added Default ctor to FileHandle.
ProtObjFactory can now create FileHandles.
Use int_codec instead of implementing it itself.
ClientHandle is now a Receivable.
ProtObjFactory can now create a ClientHandle.
Acknowledged is now a Receivable.
Test that Acknowledged::read_payload does not throw
ProtObjFactory can now create Acknowledged.
NotAcknowledged is now a Receivable.
ProtObjFactory can now create NotAcknowledged
Made test code DRY
ProtObjFactory::create now throws UnknownProtocolObjectType when an unknown type spec is encountered
Cleaned up ProtObjFactory
ProtObjFactory throws if the payload length is wrong.