Manage extra development via Gemfile.dev

This PR is just an idea about managing extra development dependencies and also solves a problem using `rake mutant`.

Before this PR `rake mutant` failed for me with:
```
/home/peter/.rvm/gems/ruby-2.1.5@global/gems/bundler-1.7.9/lib/bundler/rubygems_integration.rb:256:in `block in replace_gem': mutant is not part of the bundle. Add it to Gemfile. (Gem::LoadError)
      from /home/peter/.rvm/gems/ruby-2.1.5@pg/bin/mutant:22:in `<main>'
```

After this PR there is no need to do `rake devinit` anymore.
Just do

```bash
export BUNDLE_GEMFILE=Gemfile.dev
bundle
rake
```

Comments?