Use Reporters to better integrate info about screenshots in RSpec output.

Breaking changes:
 * `Capybara::Screenshot.append_screenshot_path` has been renamed to `Capybara::Screenshot::RSpec.add_link_to_screenshot_for_failed_examples`

User visible changes:
 * By default, RSpec’s HTML output (when using HtmlFormatter or TextMateFormatter) for failed examples now includes links to the screenshot and saved HTML, if available.
 * The `full_description` of a failed example is no longer modified to include information about screenshots (the paths / links are printed as part of the additional failure info instead). This should avoid problems with CI relying on the exact contents of `full_description`.

`HtmlEmbedReporter` propably isn’t very useful – it’s more like a proof of concept to demonstrate how easy it is to switch reporters :-)

Tested with: Ruby 1.9 / 2.0,  RSpec 2.11 / 2.14 / 3.0