title: Pip freeze does not show repository paths for requirements file
body: <p>I've created an environment and added a package django-paramfield via git:</p>
  
  <pre><code>$ pip install git+https://bitbucket.org/DataGreed/django-paramfield.git
  Downloading/unpacking git+https://bitbucket.org/DataGreed/django-paramfield.git
    Cloning https://bitbucket.org/DataGreed/django-paramfield.git to /var/folders/9Z/9ZQZ1Q3WGMOW+JguzcBKNU+++TI/-Tmp-/pip-49Eokm-build
  Unpacking objects: 100% (29/29), done.
    Running setup.py egg_info for package from git+https://bitbucket.org/DataGreed/django-paramfield.git
  Installing collected packages: paramfield
    Running setup.py install for paramfield
  Successfully installed paramfield
  Cleaning up...
  </code></pre>
  
  <p>But when i want to create a requirements file, i see only the package name:</p>
  
  <pre><code>$ pip freeze
  paramfield==0.1
  wsgiref==0.1.2
  </code></pre>
  
  <p>How can I make it output the whole string <code>git+https://bitbucket.org/DataGreed/django-paramfield.git</code> instead of just a package name? The package isn't in PyPi.</p>
  
  <p><strong>UPD</strong>: perhaps, it has to do something with setup.py? Should I change it somehow to reflect repo url?</p>
  
  <p><strong>UPD2</strong>: I found quite a similar <a href="https://stackoverflow.com/questions/6645676/pip-freeze-captures-the-package-name-as-if-it-was-on-python-index-site-but-it-i">question</a> in stackoverflow, but the author was not sure how did he manage to resolve an issue and the accepted answer doesn't give a good hint unfortunately, though judging from the author's commentary it has something to do with the setup.py file.</p>
  
  <p><strong>UPD3</strong>: I've tried to pass <code>download_url</code> in setup.py and installing package via pip with this url, but he problem persists.</p>