Begin work on adding RPC support:
  - Add the Call method for invoking a remote RPC
  - Have the HELLO message include the roles
  - Add a WaitForSession method which waits for the session WELCOME message. Callers can use this to make sure a valid session exists before starting RPC calls.
Merge pull request #52 from josephschorr/v2-rpc

Begin work on adding RPC support