Escape user name in Airbrake notify command

If a user has manually overridden this variable to be a full name, it
may contain a space which will break the command. Escaping the name
solves this issue.

This is an attempt to solve #103, which has been bothering me for almost a year. The main issue is that I don't want to use simple usernames because at a decentralized-enough company, those are basically user-chosen. I like using full, real names, but those break. This fixes it (at least in Unix).