Fix issue #22 - Allow install with Python 2.7

[clach04]

No description provided.

[clach04]

Pip install works for me with Python 2.10.18 64-bit Under Windows.

All tests pass bar one, which is relying on a website that no longer exists:

(py27venv) C:\code\py\pypub\pypub_wcember_original_upstream>python pypub/unit_tests_chapter.py
....
----------------------------------------------------------------------
Ran 4 tests in 0.326s

OK

(py27venv) C:\code\py\pypub\pypub_wcember_original_upstream>python pypub/unit_tests_clean.py
......
----------------------------------------------------------------------
Ran 6 tests in 0.010s

OK

(py27venv) C:\code\py\pypub\pypub_wcember_original_upstream>python pypub/unit_tests_epub.py
....
----------------------------------------------------------------------
Ran 4 tests in 60.727s

OK

(py27venv) C:\code\py\pypub\pypub_wcember_original_upstream>python pypub/unit_tests_image.py
..E.
======================================================================
ERROR: test_save_image (__main__.ChapterTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "pypub/unit_tests_image.py", line 24, in test_save_image
    'test image ' + str(index)),
  File "C:\code\py\pypub\pypub_wcember_original_upstream\pypub\chapter.py", line 82, in save_image
    raise ImageErrorException(image_url)
ImageErrorException: Error downloading image from http://bothsides.wpengine.netdna-cdn.com/wp-content/uploads/2015/11/bothsides1.jpg

----------------------------------------------------------------------
Ran 4 tests in 1.829s

FAILED (errors=1)