Add support for Rails 4

As discussed in #32, this pull request implements support for Rails 4, but maintains support for 3.2.

@FooBarWidget let me know if you want to have a look over this, or just want me to get it out the door.

Before releasing a stable 3.0.0 I'll also address the remaining 3 open issues, probably tomorrow. In the mean time, here's a summary of what I've changed other than the obvious things.

* Added Bundler, plus multiple Gemfiles to support testing on Rails 3.2 and 4 in a way that will work on Travis.

* Assumed this library will only support Rails 3.2 and above; people on older versions of Rails can install an older version.

* Bumped version to 3.0.0.beta1 to avoid confusion over supported Rails versions.

* Converted README to Markdown in order to get syntax highlighting on Github.

* Set required Ruby version to 1.8.7 or higher in the gemspec.