Add script for creating test virtualenv

so that tests are not reliant on `/home/vineet/.virtualenvs/equimapper`

This fixes for me all but one of the test failures in https://github.com/naiquevin/pipdeptree/issues/3

```
$ python tests/create_test_virtualenv.py
Creating test virtualenv: tests/virtualenvs/equimapper
New python executable in tests/virtualenvs/equimapper/bin/python
Installing setuptools, pip...done.
Downloading/unpacking Flask==0.10.1 (from -r tests/virtualenvs/equimapper/../equimapper_requirements.txt (line 1))
...
Successfully installed Flask Flask-Script Jinja2 Lookupy-master Mako MarkupSafe SQLAlchemy Werkzeug alembic gnureadline ipython itsdangerous psycopg2 redis slugify
Cleaning up...
```

```
$ tox -e py27
GLOB sdist-make: /Users/marca/dev/git-repos/pipdeptree/setup.py
py27 inst-nodeps: /Users/marca/dev/git-repos/pipdeptree/.tox/dist/pipdeptree-0.3.zip
py27 runtests: commands[0] | nosetests tests/
....F
======================================================================
FAIL: pipdeptree_tests.test_render_tree_freeze
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/marca/dev/git-repos/pipdeptree/.tox/py27/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/Users/marca/dev/git-repos/pipdeptree/tests/pipdeptree_tests.py", line 73, in test_render_tree_freeze
    assert '-e git+git@github.com:naiquevin/lookupy.git@cdbe30c160e1c29802df75e145ea4ad903c05386#egg=Lookupy-master' in lines
AssertionError

----------------------------------------------------------------------
Ran 5 tests in 0.245s

FAILED (failures=1)
ERROR: InvocationError: '/Users/marca/dev/git-repos/pipdeptree/.tox/py27/bin/nosetests tests/'
___________________________________________________________________________________ summary ____________________________________________________________________________________
ERROR:   py27: commands failed
```

So only one failure now instead of five!