Gemfile + Test on multiple rails versions

A few additions, hope you like them :)

 - Make tests run via bundler / do not allow other dependencies
 - Test on 3.2 and 4.0 when running rake
 - test on preferred version when running `BUNDLER_GEMFILE=gemfiles/32.gemfile rake test`
 - remove unnecessary Concern complexity (not necessary / imo makes code more complex / replaced by include InstanceMethods)

If you also want travis then add something like this:
```
#.travis.yml
rvm:
  - 1.8.7
  - 1.9.3
  - 2.0
gemfile:
  - gemfiles/32.gemfile
  - gemfiles/40.gemfile
bundler_args: --no-deployment
script: "rake test"
```
@nthj