Add Tox: Include tox for multiversion testing

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:
```
$ tox

|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 :)
```