Clean up server slightly

* Handle all arg parsing in server.js

* wrap the server into a constructor taking options and call listen inside it,
  so that the ParsoidService is self-contained.

* make the -c option an alias for --config and add the -n/--num-workers
  option instead

* renamed ParserService.js to ParsoidService.js for consistency

* updated doc comments in server.js and ParsoidService.js

Change-Id: I5fb2aba5c3b54e829eb71c58d44e422db5375122
Merge "Clean up server slightly"