[MRG] switch to pytest

In this PR I've switched testing suite from nose to pytest. 

The biggest change is that the testservers fixture is now explict (no more global variables). This allows to run tests in parallel using pytest-xdist; nose parallelization didn't work for me for a misterious reason. Also, for some reason py.test runs tests faster on OS X even single-threaded - maybe converting fixtures to explicit fixed some subtle issue with testing setup. 

Tests that used to take 200-300s now run in 30-40s on my notebook using `py.test -n4`.