(For Jacob) Make capistrano-slack code better.

@jburnim \
Can each of these just be def'd, without any need for pre-initializing all of these class methods? -- e.g.:
```ruby
def deploy_failed_msg
  announcement = "#{announced_deployer} cancelled deployment of #{slack_application}"
  announcement << "'s #{fetch(:branch)}" if fetch(:branch, nil)
  announcement << " to #{announced_stage}"
end
```