Add pytest Test Suite

[peterstory]

• Setup instructions
• A test for search()

For #44

Here is an example of the results of running the test suite:

(venv) bash-3.2$ python -m pytest -v
================================== test session starts ==================================
platform darwin -- Python 3.5.4, pytest-3.4.2, py-1.5.2, pluggy-0.6.0 -- /Users/pstory/Documents/git_repos/googleplay-api/venv/bin/python
cachedir: .pytest_cache
rootdir: /Users/pstory/Documents/git_repos/googleplay-api, inifile: pytest.ini
collected 1 item                                                                        

test/test_googleplay.py::TestGooglePlay::test_search PASSED                       [100%]

=============================== 1 passed in 4.99 seconds ================================

I find unit-testing with pytest to be an invaluable part of the development process, so I think you guys will appreciate using it, too!