Mark cleaned strings as html safe

Due to a technical limitation, I'm unable to mark strings that call `Sanitize::Rails::Engine.clean!` as html_safe; there is no clean way to change an instance of `String` in place into an instance of `ActiveSupport::SafeBuffer`. That being said, `Sanitize::Rails::Engine.clean` now marks any text passed through it as html_safe!

Additionally, this branch stubs out a new test suite. For now it just verifies the behavior of `Sanitize::Rails::Engine` and the `Sanitize::Rails::String` extension, but I think that's comprehensive enough to give me confidence that the changes presented here won't break anything.

Lastly, I've initialized a TravisCI configuration. Once this is merged in you should see CI builds start to pop up under https://travis-ci.org/vjt/sanitize-rails. And yes, a build status badge has indeed been added to the README. Enjoy!

https://github.com/damien/sanitize-rails/tree/4-mark-cleaned-strings-as-html-safe: [![Build Status](https://travis-ci.org/damien/sanitize-rails.png?branch=4-mark-cleaned-strings-as-html-safe)](https://travis-ci.org/damien/sanitize-rails)