Installer for decl_auth

## Updates ##
* Adds generator for default auth rules: `rails g authorization:rules`
* Adds installer for quick setup
  * `rails g authorization:install [USERMODEL] [field:type field:type ...] [options]`
  * First argument is the name of the model to add roles to.  By default it's `User`
  * Other arguments are attributes for the model generator if `--create-user` is passed
  * `--commit` option to run `rake db:migrate` and `db:seed`
  * `--create-user` option to create user model defined in params
  * `--user-belongs-to-role` option to make users have only one role.  By default it's HABTM
* Updates README with Quick Start guide