Added common credentials files and directories containing personal information

* Added common credential files to `Rails.gitignore` (ex: `config/mailer.yml` and `config/paypal.yml`).
* Added `Dotfiles.gitignore` for when someone wishes to create a dotfiles repository for their home directory:
  * Added common credential files/directories (ex: `.ssh/`).
  * Added directories containing transient user data (ex: `Downloads/`).
  * Added directories containing personal information and/or credentials (ex: `.mozilla/`)
  * Added directories containing user installed software (ex: `.gem/`).
  * Added history files (ex: `.bash_history`).
  * Added transient directories and files (ex: `.cache/`).
  * Whitelisted safe configuration files (ex: `.ssh/config`).