Add Tox: Include tox for multiversion testing

PPACL-2  

Add tox.ini file to enable multiversion testing.  The done state for this card is to enable tox for python version 2.6 only.  An additional card in this sprint will cover adding python version 2.7, and 3.3.

For testing, 
get the latest build and this branch
run in a virtualenv with python 2.6 installed
run the command 
$ tox 

the results of executing the above command and verifying passing tests should be this:
(AWeber-API-Python-Library)miked@dev-mbp-miked-3:~/git/AWeber-API-Python-Library (add-tox*+) $ tox

GLOB sdist-make: /Users/miked/git/AWeber-API-Python-Library/setup.py
py26 inst-nodeps: /Users/miked/git/AWeber-API-Python-Library/.tox/dist/aweber_api-1.1.4.zip
py26 runtests: commands[0] | python setup.py nosetests
running nosetests
running egg_info
writing requirements to aweber_api.egg-info/requires.txt
writing aweber_api.egg-info/PKG-INFO
writing top-level names to aweber_api.egg-info/top_level.txt
writing dependency_links to aweber_api.egg-info/dependency_links.txt
reading manifest file 'aweber_api.egg-info/SOURCES.txt'
reading manifest template 'MANIFEST.in'
writing manifest file 'aweber_api.egg-info/SOURCES.txt'
running build_ext
.............................................................................
Name                    Stmts   Miss  Cover   Missing
-----------------------------------------------------
aweber_api                 59     14    76%   28-46, 57, 90, 95
aweber_api.base            29      0   100%   
aweber_api.collection      86     16    81%   43-45, 48-58, 101-102, 127, 131
aweber_api.data_dict       10      0   100%   
aweber_api.entry           83      0   100%   
aweber_api.oauth           68      9    87%   21, 23-25, 36-39, 70, 75
aweber_api.response        21      1    95%   29
-----------------------------------------------------
TOTAL                     356     40    89%   
----------------------------------------------------------------------
Ran 77 tests in 0.421s

OK
summary _________________________________________________________________
  py26: commands succeeded
  congratulations :)