Now using py.test for unit testing

- Also added some tags and signal names to builder
  for easier debugging in some cases
- Running with multiple simulators and neuron types
  is done with py.test's funcargs mechanism;
  see http://pytest.org/latest/funcargs.html
  - Additional neuron types are tested in several cases
- Some renaming and cleanup in tests
- python setup.test still works with setuptools,
  but not with distutils. Install should still work though.
Renamed white_noise to whitenoise
Removing .py examples

- I'll add a function to generate these from the .ipynbs
Added PEP8 to testing via pytest-pep8
Added docstring for Simulator

- This shows up when doing py.test --fixtures
Using pytest.mark.parametrize in test_ensemble
Now using tox for testing

This makes it easy to know if your changes will work on Python 2.6,
2.7 and 3.3 without having to push up to travis-ci.

Now, if you pip install tox, you don't need anything else installed;
you can just run `tox` and it will set up a 2.6, 2.7, and 3.3
environment for testing. tox will work just fine in a virtualenv too,
so no changes required if you already have a virtualenv set up.

- .travis.yml uses tox now as well
IPython notebook examples now checked

- Make sure they have no output
- Make sure they don't raise exceptions
- Right now, marked as expected failures
python setup.py test now uses tox
Updated all examples to new syntax

- They all run without exceptions on 2.6, 2.7 and 3.3