Squashed a minor commit to review fixes.

Added fixes on review comments on pull req #3. Mostly minor.

Importing only whats needed.

Added comment to explain socket options zmq.REQ_CORRELATE and zmq.REQ_RELAXED on REQ socket in client.

Added todo about handling string length while implementing doc chunking.

Added todo about parallelizing send document and get document operations.

Doc indentation fix.

Fixed typo in doc.

Updated todo about moving target_id etc. to a target_settings file.

Updated import styles in zmq_client, zmq_server and zmq_target modules.

Added todo about handling DecodeError and trying to recover sync.

Changed numbers in identifier type to named constants.

Changed manual assignment of named constants in protobuf_settings to make it assign from proto.Identifier.Type enums.

Doc fixes in utils.py

More doc fixes.

Added todo about completing handle_put_sync_info_request function.

Updated todo about removing unneeded deps from u1db.__init__

Removed assert statement.

More doc fixes.

Minor typo fixes.