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

#### test results
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 :)