Support Python 2.6

Unfortunately I can't get the tests to pass because of https://github.com/naiquevin/pipdeptree/issues/3

```
 ❯ tox -e py26
GLOB sdist-make: /Users/marca/dev/git-repos/pipdeptree/setup.py
py26 create: /Users/marca/dev/git-repos/pipdeptree/.tox/py26
py26 installdeps: nose
py26 inst: /Users/marca/dev/git-repos/pipdeptree/.tox/dist/pipdeptree-0.3.zip
py26 runtests: commands[0] | nosetests tests/
EEFFF
======================================================================
ERROR: pipdeptree_tests.test_req_version
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/marca/dev/git-repos/pipdeptree/.tox/py26/lib/python2.6/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 28, in test_req_version
    sqlalchemy = find_req('sqlalchemy', 'alembic')
  File "/Users/marca/dev/git-repos/pipdeptree/tests/pipdeptree_tests.py", line 24, in find_req
    return [r for r in pkg_index[parent].requires() if r.key == req][0]
IndexError: list index out of range

======================================================================
ERROR: pipdeptree_tests.test_non_top_pkg_name
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/marca/dev/git-repos/pipdeptree/.tox/py26/lib/python2.6/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 36, in test_non_top_pkg_name
    flask_r = find_req('flask', 'flask-script')
  File "/Users/marca/dev/git-repos/pipdeptree/tests/pipdeptree_tests.py", line 24, in find_req
    return [r for r in pkg_index[parent].requires() if r.key == req][0]
IndexError: list index out of range

======================================================================
FAIL: pipdeptree_tests.test_render_tree_only_top
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/marca/dev/git-repos/pipdeptree/.tox/py26/lib/python2.6/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 52, in test_render_tree_only_top
    assert '  - SQLAlchemy [required: >=0.7.3, installed: 0.9.1]' in lines
AssertionError

======================================================================
FAIL: pipdeptree_tests.test_render_tree_list_all
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/marca/dev/git-repos/pipdeptree/.tox/py26/lib/python2.6/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 62, in test_render_tree_list_all
    assert '  - SQLAlchemy [required: >=0.7.3, installed: 0.9.1]' in lines
AssertionError

======================================================================
FAIL: pipdeptree_tests.test_render_tree_freeze
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/marca/dev/git-repos/pipdeptree/.tox/py26/lib/python2.6/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 72, in test_render_tree_freeze
    assert '  - SQLAlchemy==0.9.1' in lines
AssertionError

----------------------------------------------------------------------
Ran 5 tests in 1.044s

FAILED (errors=2, failures=3)
ERROR: InvocationError: '/Users/marca/dev/git-repos/pipdeptree/.tox/py26/bin/nosetests tests/'
___________________________________________________________________________________ summary ____________________________________________________________________________________
ERROR:   py26: commands failed
```