Big Commit. The old Register has been renamed Initialize, the new 
Register is used by components to inform other components that they 
have finished initializing. DocumentCheckActor now will ensure a line 
has registered before sending a passenger to that line. NewPassenger is
a new message class used when sending a new passenger to the document
check. 

Known issues: Sending a new passenger to the doc check before any lines
have been registered results in an error message and the passenger 
being lost by the system. This does NOT cause a crash, however.