Add Heroku scripts

staging:

    staging console
    staging migrate
    staging process
    staging open
    watch staging ps
    staging tail

production:

    production backup
    production console
    production migrate
    production open
    watch production ps
    production tail

load-backup-into:

    load-backup-into staging
    load-backup-into development

Notes on the last command:

* Copy latest production database backup into development database using
  preferred pgbackups + pg_restore method recommended by Heroku.
* Do not assume local development database name.
* Get local database name from config/database.yml.
* This did not previously exist in dotfiles.

Supporting change for these commands:

* Add ~/.bin to the PATH.

Unrelated changes:

* Detect presence of Zeus for rk script.
* Use Zeus if present when migrating.
* Use one-letter aliases for super-common Rails commands.