Fix urls to links

Trying to fix #129 I added a couple of gems: `rspec`, `rake` and `rack-test`.

I used `rspec` to test `urls_to_links`, it's also used to test `pbkdf2` but the gem wasn't included in the `Gemfile`.

`rake` and `Rakefile` helps running specs, the command is `bundle exec rake`.

I included also `rack-test` gem (see http://www.sinatrarb.com/testing.html) even if it's not used at the moment.