alias given and given! to let and let!

[danryan]

I use let all over the place in vanilla RSpec, but something just didn't feel right using it with the feature DSL. This pull request adds aliases for let and let! in the form of given and given!. It's only syntactic sugar but it helps remind me I'm writing acceptance tests.

Using these aliases, the DSL example in the README could be rewritten as:

feature "Signing up" do
  given(:user) { User.make(:email => 'user@example.com', :password => 'caplin') }

  scenario "Signing in as another user" do
    within("#session") do
      fill_in 'Login', :with => user.email
      fill_in 'Password', :with => user.password
    end
    click_link 'Sign in'
  end

end

[jnicklas]

I like it!

[fabianoalmeida]

That's great @danryan. Congratulations! Simple and useful.