Add support for tox

```
 ❯ tox
GLOB sdist-make: /Users/marca/dev/git-repos/gsocketpool/setup.py
py26 inst-nodeps: /Users/marca/dev/git-repos/gsocketpool/.tox/dist/gsocketpool-0.1.2.zip
py26 runtests: commands[0] | nosetests
.......................
----------------------------------------------------------------------
Ran 23 tests in 2.992s

OK
py27 inst-nodeps: /Users/marca/dev/git-repos/gsocketpool/.tox/dist/gsocketpool-0.1.2.zip
py27 runtests: commands[0] | nosetests
.......................
----------------------------------------------------------------------
Ran 23 tests in 2.893s

OK
___________________________________________________________________________________ summary ____________________________________________________________________________________
  py26: commands succeeded
  py27: commands succeeded
  congratulations :)
```