Make it compatible with RSpec 3.

From the release notes of RSpec 3:

> Remove `RSpec::Core::ExampleGroup#example` and `RSpec::Core::ExampleGroup#running_example` methods. If you need access to the example (e.g. to get its metadata), use a block arg instead
Fix missing 'not' in spec name.
Implement `Saver#screenshot_saved?` and `Saver#html_saved?`

So we can later improve the output to only include the screenshot’s filename if a screenshot was actually saved.
Add screenshot path to RSpec metadata only if a screenshot was actually saved.
Add path to saved HTML to RSpec metadata, if `append_screenshot_path` is set.

Only if the HTML has actually been saved, of course.

This makes the warning in `Saver#save_html` obsolete, so I removed it.
Remove warnings for drivers not supporting screenshots.

I don’t think these warnings are necessary, and they clutter RSpec’s output if e.g. rack_test is used deliberately for some specs.
Merge pull request #75 from noniq/fix-for-rspec3

Make it compatible with RSpec 3.
Merge pull request #76 from noniq/improve-output

Improve output and reduce clutter