Major changes to the deployer plugin:

- The plugin is now configurable. You can now specify the path to your ansible
  playbook executable, the directory where your ansible data lives, and a mapping
  of app names to playbooks.

- Took the first step toward making the app/playbook combo controllable by
  making the app the first parameter to `execute()`.

- Cleaned up the chat output considerably. The bot now only echoes successful
  tasks and other major steps in the run. It provides slightly more detail on
  fatal errors.

- Switched from `buffer` to gnu `stdbuf` for unbuffering ansible output.
  Instructions for installing stdbuf with homebrew on OS X are in the
  comments in the file header.