Travis CI Integration

* Add `Rake::TestTask` to `Rakefile` and set default task tol `:test`
* `CONTRIBUTING.md` notes test dependencies and how tests are run
* In bundler `:debug` group, conditionally install ruby-debug for 1.8 and debugger for 1.9
* Add `test_helper.rb` which is required by all tests
* `test_helper.rb` conditionally requires debugging gem
* Add Travis CI configuration
* Per Travis CI recommendation, do not install debugging gems on CI