DOC: testing how-to use fixtures / parametrize
I think might be nice to update: http://pandas-docs.github.io/pandas-docs-travis/contributing.html#writing-tests

with how to make / use fixtures and how to use parametrize. something like (and mention that we cannot do this in a ``class TestClass(...)`` structure)

Its nice for a beginner to write this up because it will then be from there perspective :>
```
(pandas) bash-3.2$ more tester.py
import pytest
import numpy as np
import pandas as pd


@pytest.mark.parametrize('dtype', ['int8', 'int16', 'int32', 'int64'])
def test_dtypes(dtype):
    assert str(np.dtype(dtype)) == dtype

@pytest.fixture
def series():
    return pd.Series([1, 2, 3])

@pytest.fixture(params=['int8', 'int16', 'int32', 'int64'])
def dtype(request):
    return request.param

def test_series(series, dtype):
    series = series.astype(dtype)
    assert series.dtype == dtype
```

and running it
```
((pandas) bash-3.2$ pytest  tester.py  -v
============================================================================================== test session starts ===============================================================================================
platform darwin -- Python 3.5.2, pytest-3.0.5, py-1.4.31, pluggy-0.4.0 -- /Users/jreback/miniconda3/envs/pandas/bin/python
cachedir: .cache
rootdir: /Users/jreback/pandas, inifile: setup.cfg
plugins: cov-2.4.0, xdist-1.15.0
collected 8 items 

tester.py::test_dtypes[int8] PASSED
tester.py::test_dtypes[int16] PASSED
tester.py::test_dtypes[int32] PASSED
tester.py::test_dtypes[int64] PASSED
tester.py::test_series[int8] PASSED
tester.py::test_series[int16] PASSED
tester.py::test_series[int32] PASSED
tester.py::test_series[int64] PASSED
```

and lastly about using ``tm.assert_series_equal`` and such.