run tests with py.test and include doctests

 - conftest.py to keep from sourcing setup.py
 - pytest.ini to always include doctests with running tests

This PR will run all of the tests via pytest, including doctests found in python modules.
It provides the same functionality via the test method (i.e. 'import datashape; datashape.test())
The pytest.ini file will make it so that any command line call to py.test will automatically run the doc tests. 
It keeps Travis CI running the same line as always (which is actually nice if we want to add functionality, .travis.yml doesn't have to change)