Adding spring

Commands for rails, rake and rspec have been modified to be automatically prefixed with spring and reside in the `"#{Rails.root}/bin"` directory.

In order to avoid prefixing every command with `bin/` you can use [direnv](https://github.com/zimbatm/direnv).

In OSX the following commands have to be issued to install direnv for spring with rails:

1.  `$brew install direnv`
2.  `eval "$(direnv hook bash)"` or `eval "$(direnv hook zsh)"` must be added to "~/.bashrc" or "~/.zshrc"
3.  `$direnv edit .` from Rails application root directory
4.  add `export PATH="./bin:$PATH"` into the `.envrc` file which is automatically opened and save the file