Let's start testing against Rails 3 and Rails 4

This pull request sets up [Appraisal](https://github.com/thoughtbot/appraisal) to test against multiple versions of Rails, and also contains a [Travis.ci](https://travis-ci.org/binarylogic/authlogic) config against multiple versions of Ruby.

A first step towards Rails 4 support. Yes, there are **test failures**. That's kind've the point.

```
rake appraisal:install
rake appraisal
```