Add rspec/its to run specs

`its` has been extracted to the `rspec-its` gem:

http://myronmars.to/n/dev-blog/2014/05/notable-changes-in-rspec-3#removals
Provide delegators for the 'id' and 'message' keys

That are returned when using a `callback_url`, or there is an error.
Add failing spec for callback_url provided as an option
Fix issue preventing callback_url to be provided as an option

If a `callback_url` were provided in the following manner:

    kraken.url('http://example.com/foo.jpg', :callback_url =>
'http://example.com/kraken_results')

Then this query:

    params.keys.include?(:callback_url)

would be false, because `keys` returns an array of strings (not symbols).

Also, if that were corrected, and @callback_url is nil, then

    params[:callback_url] = @callback_url

would set the callback_url to nil, and the resulting params sent to
Kraken would not have either a `callback_url` or a `wait` option.
Update README

Changes include:

* Using Gemfile-aware installation instructions
* Introduce authentication section earlier to setup kraken instance
* Remove boilerplate to setup the kraken instance in every example
* Add notes on wait/callback_url below url & upload sections
* Add tip to getting started to sign up for developer plan to test integration
* Move Downloading Images section below examples, and show open-uri example
* Replace javascript request/response examples with ruby ones
* Use message calling (dot syntax) for response “data” objects
* Show callback_url endpoint as typical Rails-ish controller action
* Minor language tweaks