title: preventing python coverage from including virtual environment site packages
body: <p>I am new to coverage and ran into a strange problem. My coverage is taking my virtual environment site packages into account.
  Here is the output of the coverage run:</p>
  
  <pre><code>coverage run test.py
  ....................
  ----------------------------------------------------------------------
  Ran 20 tests in 0.060s
  
  OK
  (atcatalog)- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -jmfrank63@fullstack-audio-text-catalog:~/workspace (git master)       [19:58:45]
  $ coverage report
  Name                                                                              Stmts   Miss  Cover
  -----------------------------------------------------------------------------------------------------
  /home/ubuntu/Envs/atcatalog/lib/python2.7/site-packages/flask/__init__               18      0   100%
  /home/ubuntu/Envs/atcatalog/lib/python2.7/site-packages/flask/_compat                38     20    47%
  /home/ubuntu/Envs/atcatalog/lib/python2.7/site-packages/flask/app                   528    255    52%
  /home/ubuntu/Envs/atcatalog/lib/python2.7/site-packages/flask/blueprints            156    118    24%
                               .
                               .
                               .
  /home/ubuntu/Envs/atcatalog/lib/python2.7/site-packages/werkzeug/urls               412    215    48%
  /home/ubuntu/Envs/atcatalog/lib/python2.7/site-packages/werkzeug/utils              242    175    28%
  /home/ubuntu/Envs/atcatalog/lib/python2.7/site-packages/werkzeug/wrappers           568    298    48%
  /home/ubuntu/Envs/atcatalog/lib/python2.7/site-packages/werkzeug/wsgi               448    352    21%
  atcatalog/__init__                                                                    7      0   100%
  atcatalog/views/__init__                                                              0      0   100%
  atcatalog/views/publang                                                               7      0   100%
  atcatalog/views/pubtext                                                               1      0   100%
  atcatalog/views/userlang                                                             13      0   100%
  atcatalog/views/users                                                                 5      0   100%
  atcatalog/views/usertext                                                             14      0   100%
  test                                                                                120      0   100%
  -----------------------------------------------------------------------------------------------------
  TOTAL                                                                             12530   8044    36%
  (atcatalog)- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -jmfrank63@fullstack-audio-text-catalog:~/workspace (git master)       [19:58:55]
  </code></pre>
  
  <p>Here is the structure of my project directory which resides under home:</p>
  
  <pre><code>workspace/
  ├── README.md
  ├── atcatalog
  │   ├── __init__.py
  │   ├── __init__.pyc
  │   ├── static
  │   ├── templates
  │   └── views
  │       ├── __init__.py
  │       ├── __init__.pyc
  │       ├── publang.py
  │       ├── publang.pyc
  │       ├── pubtext.py
  │       ├── pubtext.pyc
  │       ├── userlang.py
  │       ├── userlang.pyc
  │       ├── users.py
  │       ├── users.pyc
  │       ├── usertext.py
  │       └── usertext.pyc
  ├── requirements.txt
  ├── run.py
  └── test.py
  </code></pre>
  
  <p>I had the virtual environment at first inside the project directory and now moved it out to ~/Envs with virtualenvwrapper, but the problem persisted.
  run.py and test.py are not special in any way, they both import app from atcatalog.
  I also tried to find ways to omit the virtual environment directory, but google gave no answer (surprisingly).
  I don't think it is the purpose of coverage to test already well tested site-packages. So I would exclude them from the run.</p>
  
  <p>How can I accomplish to avoid coverage having testing my site-packages?</p>