Feature/change test layout

[dmitrypolo]

This addresses #80 .

1. Puts all fixtures into the conftest.py file as recommended by pytest.
2. Adds a pytest.ini file to ignore RuntimeWarning when running pytest. This comes about when running older versions of numpy and sklearn. The output of the tests remains as intended.
3. Creates a function in impyute/util/testing.py which does the two tasks of checking the return_type and missing_value which appeared in almost every test file.
4. Modified the tests to use these fixtures as well as other pytest best practices to make code more readable.
5. Fixed the test_compare.py test to actually test the output of the file. Previously it wrote out a file to the repo that would remain there. Now it uses pytest fixtures to write to a temporary directory, check the contents of the file, and then delete the file so that the user does not have to have this file hanging around their system.
6. Changed the last two remaining test files using unittest to use pytest so that testing is consistent across the whole package.

[eltonlaw]

Wow this looks great! Thanks a lot!