Remove 3.1 and 3.2 from setup.py, add tox.ini to simplify testing on multiple versions

This PR removes Python 3.1 and 3.2 from the trove classifiers in `setup.py` since @kennethreitz decided to drop support for those versions in the rejected PR #1326. 

It also introduces a rudimentary "tox.ini" which makes local tests with multiple Python versions trivial:
```
$ pip install tox
$ cd requests-sources/
$ tox
GLOB sdist-make: setup.py
py26 inst-nodeps: .tox/dist/requests-1.2.0.zip
py26 runtests: commands[0]
.........................................
Ran 41 tests in 24.071s
OK
py27 inst-nodeps: .tox/dist/requests-1.2.0.zip
py27 runtests: commands[0]
.........................................
Ran 41 tests in 23.269s
OK
py33 inst-nodeps: .tox/dist/requests-1.2.0.zip
py33 runtests: commands[0]
.........................................
Ran 41 tests in 22.674s
OK
________________ summary ________________
  py26: commands succeeded
  py27: commands succeeded
  py33: commands succeeded
  congratulations :)
```