Rails generator

[mostlyobvious]

Add generator bbq:install as advertised in README. This should:

• guide user to add bbq/{unit,spec} to {test,spec}_helper.rb
• create {test,spec}/acceptance/
• add rake task to run acceptance tests, i.e rake {test,spec}:acceptance

Optionally it would be nice to generate TestUser stub in {test,spec}/support/test_user.rb inheriting after Bbq::TestUser and having register/login/logout actions if some auth system is present (Devise will be good for start).