Add tox.ini; make .travis.yml use it

Easy to use.

```
$ pip install tox
$ tox
...
  py26: commands succeeded
  py27: commands succeeded
ERROR:   py33: commands failed
ERROR:   py34: commands failed
```

Easy.

I changed `.travis.yml` to leverage tox so that the configuration is not repeated too much between the two files. As you can see, Travis CI still works:

https://travis-ci.org/msabramo/smmap/builds/25342009

I hope that this is useful for you.