Methods and test suite update

## Test suite updates
* Move from minitest to rspec
* Add capybara-webkit for headless tests
* Use spork to fasten the tests of the development environment
* Remove the gems from the Gemfile. Now using the gemspec method
* Update all the tests to use the rspec expect syntax
* Now adding the VCR cassettes to the test suite

## Methods update
* Add update method to recurrent contributions
* Add get_executions method to recurrent contributions
* Add upload method to Strong Authentication
* Add the form_request method to Ressources

## Gem Update
* Move to version 1.1.0
* Update documentation to reference [mangopay](http://www.mangopay.com)
* Add some support files to test file uploads