Update .gitignore with *.pyc
Finish testing PersistenceEngine
Finish testing PersistenceStage
Add __init__.py
Refactor for use of util.Key and util.Value
Add Key, Value, ErrorCode classes.
Add use of util.Key to tests
Add __hash__ to Key
Add test template
Add from_node_list, get_node_address

Yet to implement partition_keyspace
Add skeleton files for request stages.

InternalRequestStage and ExternalRequestStage
Add methods for consistent hash ring
Empty test file for ConsistentHashRing