Allow dynamic urls in page objects

Example:

```ruby

# UserProfile.rb
class UserProfile < SitePrism::Page
  set_url { "/users/#{@username}" }
  set_url_matcher { url }
  def initialize(username)
     @username = username
  end
end

# user profile steps
When /I visit Bob's profile/ do
  @bob = UserProfile.new('bob')
  @bob.load  # -> /users/bob
end
Then /I should see Bob's profile/ do 
  @alice = UserProfile.new('alice')
  @bob.should be_displayed  #=~ /users/bob is true
  @alice.should_not be_displayed #=~ /users/alice is false
end
```