Add current_fullpath method that returns path with query string.

As redundant as `current_path` and `current_host` (URI + `current_url`), but a more common use case than `current_host`, so I think that the convenience is worth it.

80 votes on SO: http://stackoverflow.com/questions/5228371/how-to-get-current-path-with-query-string-using-capybara/25999027#25999027

I will add tests if the feature is accepted.