Added a #operation method to Seahorse::Client::Base.

This method accepts an operation name and returns the
`Seahorse::Client::Model::Operation` object named or
raises an error when it can not be found.
Attempting to enable integration tests via travis.

You can now enable integration tests two ways.

* Create a integration-test-config.json file in the project root
* export AWS_INTEGRATION

Added the AWS_INTEGRATION and necessary secure env variables
to .travis.yml to enable integration tests on Travis-CI.  Also
moved the logic for running/skipping integration tests out of the
rake test.rake file and into the cucumber env.rb.