Add tox.ini for tox and .travis.yml for Travis CI

This adds support for the [tox](http://tox.testrun.org/) testing tool, which lets you run your tests across a bunch of virtualenvs with different versions of Python.

```
$ tox
GLOB sdist-make: /Users/marca/dev/git-repos/twiggy/setup.py
py26 inst-nodeps: /Users/marca/dev/git-repos/twiggy/.tox/dist/Twiggy-0.4.5.zip
py26 runtests: PYTHONHASHSEED='1438598325'
py26 runtests: commands[0] | py.test --tb=short tests/
=================================================================================================================================================== test session starts ====================================================================================================================================================
platform darwin -- Python 2.6.7 -- py-1.4.20 -- pytest-2.5.2
collected 141 items

tests/test_converter.py .................
tests/test_filters.py .............
tests/test_formats.py ........
tests/test_globals.py ......
tests/test_integration.py .
tests/test_levels.py s..........
tests/test_lib.py ..
tests/test_logger.py ......................................
tests/test_logging_compat.py ................
tests/test_message.py .......................
tests/test_outputs.py ......

========================================================================================================================================== 140 passed, 1 skipped in 0.69 seconds ===========================================================================================================================================
py27 inst-nodeps: /Users/marca/dev/git-repos/twiggy/.tox/dist/Twiggy-0.4.5.zip
py27 runtests: PYTHONHASHSEED='1438598325'
py27 runtests: commands[0] | py.test --tb=short tests/
=================================================================================================================================================== test session starts ====================================================================================================================================================
platform darwin -- Python 2.7.6 -- py-1.4.20 -- pytest-2.5.2
collected 141 items

tests/test_converter.py .................
tests/test_filters.py .............
tests/test_formats.py ........
tests/test_globals.py ......
tests/test_integration.py .
tests/test_levels.py s..........
tests/test_lib.py ..
tests/test_logger.py ......................................
tests/test_logging_compat.py ................
tests/test_message.py .......................
tests/test_outputs.py ......

========================================================================================================================================== 140 passed, 1 skipped in 0.64 seconds ===========================================================================================================================================
pypy inst-nodeps: /Users/marca/dev/git-repos/twiggy/.tox/dist/Twiggy-0.4.5.zip
pypy runtests: PYTHONHASHSEED='1438598325'
pypy runtests: commands[0] | py.test --tb=short tests/
=================================================================================================================================================== test session starts ====================================================================================================================================================
platform darwin -- Python 2.7.6[pypy-2.3.0-alpha] -- py-1.4.20 -- pytest-2.5.2
collected 141 items

tests/test_converter.py .................
tests/test_filters.py .............
tests/test_formats.py ........
tests/test_globals.py ......
tests/test_integration.py .
tests/test_levels.py s..........
tests/test_lib.py ..
tests/test_logger.py ......................................
tests/test_logging_compat.py ................
tests/test_message.py .......................
tests/test_outputs.py ......

========================================================================================================================================== 140 passed, 1 skipped in 1.85 seconds ===========================================================================================================================================
_________________________________________________________________________________________________________________________________________________________ summary __________________________________________________________________________________________________________________________________________________________
  py26: commands succeeded
  py27: commands succeeded
  pypy: commands succeeded
  congratulations :)
```

tox is particularly useful when porting to Python 3 (https://github.com/wearpants/twiggy/issues/26), which I'd like to help out with.

I also added support for Travis CI. Here's a passing Travis CI build for my fork:

https://travis-ci.org/msabramo/twiggy/builds/21532367

To set up Travis CI for your repo (takes less than 1 minute), see http://docs.travis-ci.com/user/getting-started/