Bundler auto-detection mode - similar to check_for_rvm and check_for_rbenv

As discussed in #157 I have prepared auto-detection of [Bundler](http://bundler.io/) availability in project. The check is made for presence of `Gemfile` in a folder one level above of the `spec`, `features` or `test` folder which can be defined in `ruby_(unit|rspec|cucumber)_folder` settings. 

It's setted by default to `false` in `check_for_bundler` settings. 

I have also written a bit in the `README.md` about it.