[Python] Improve style/usage of pytest and properly adopt their idioms/structures

[suntzu86]

This is a tracking ticket. #36 got us into using pytest. Now we should actually adopt its best-practices. I'm going to throw up a few examples now but others should add more as they run across them.

apiref outlines what it can do:
http://pytest.org/latest/apiref.html
some examples:
http://pytest.org/latest/example/index.html
common cases:
https://github.com/Yelp/MOE/issues/new

Related:
#407
#409
#410
#411

[suntzu86]

http://pytest.org/latest/plugins.html

conftest.py could be a good place for some of these like #407 and #409