Add defined? guard and better custom exemple

[allaire]

We had some problems with the custom class overwrite because Capistrano and Slackistrano are only in the Gemfile development group, and since the file is in lib/, rails was trying to load it, leading to uninitialized constant Slackistrano::Messaging

I also added an exemple to use the git user full name, which is cleaner in my opinion (still falling back to the user ENV).

[phallstrom]

Thanks @allaire!