Moved from testify to pytest

********* PEOPLE *************
Primary reviewer: @suntzu86 

Reviewers:

********* DESCRIPTION **************
Branch Name: sclark_36_testify_to_pytest
Ticket(s)/Issue(s): Closes #36

-transitioned from testify to pytest, nothing fancy yet.
-single fcn vs class setup/teardown being handled properly
-fixed some scoping issues with instance v class v static methods
-applied hidetraceback where appropriate

-future cleanup/pytest improvements (it's a lot more powerful than testify!) to be tracked in #408 

********* TESTING DONE *************
make test
make style-test