Refactor the code to simplify it, increase performance, and fix 2 bugs

- Simplified the code so it's easier to read
- Return sooner if the referer is blank or the query string is blank which should slightly increase performance
- SearchReferers and StopWords don't need to be created in the initialize method (performance increase?)

The refactoring solves 2 bugs present in the original code

1. If the referer query string has spaces in it URI.split fails
2. When query_args was nil (from URI.split failing) the "unless query_args.empty?" line was failing and throwing an exception causing Rails to throw a 500 error

#1 is now properly handled by not using URI.split and #2 doesn't exist with the refactored code.