Trial functionality for running tests with pytest

  * Change current dir to tmp dir on each test run
  * Log twisted with test.log
Add py33 environment to allowed failures in travis-ci
Support doctest and __init__.py test discover in pytest
Ignore files with import errors on pytest test discover
Change function name so it does not mess up with pytest autodiscover
Fix httpcache doctest that assumed dictionary order
Ensure spiders module reload between spider manager tests