text: Title: extra_requires[dev] does not exist
  Body: Issue: running `pip install -e .[dev]` works, but doesn't install the dev requirements. this is due to the missing extra, dev
  
  ```
  pip install .[dev]
  Processing /Users/jchittum/dev01/flask-restplus
    flask-restplus 0.12.2.dev0 does not provide the extra 'dev'
  ```
  
  clearly missing in setup.py
  
  ```
      extras_require={
          'test': tests_require,
          'doc': doc_require,
      },
  ```
  
  Also, develop.pip contains an entry that is not compatible with installing in this fashion (even with the very useful parser in setup.py)
  
  ```
  -e .[test]
  invoke==0.21.0
  flake8==3.5.0
  readme-renderer==17.2
  tox==2.9.1
  ```
  