Make "911" and "get" consistent

Fixes https://github.com/stripe-contrib/pagerbot/issues/12

The explicit 'subject'/'because' is removed from documentation, but people can still use them for backward compatibility.

Caveats:
1. The message parsing code is no longer local (not fully stack-based) because it has to check if the word 'subject' or 'because' is present in ```query[:words]```
2. Without an explicit 'subject'/'because' it won't recognize when the person's name consists of two words. This is an intended behavior.
3. The parser won't be able to tell if the "subject" in "help, mr. subject is drowning!" is part of the subject, not a keyword.