tweak .coveragerc

- note source is dbsake
- exclude enum.py from coverage
add TemporaryDirectory to tests.util
update tests/mysql/test_sandbox

- use TemporaryDirectory for staging downloads / sandboxes
- suppress stderr/stdout from start/stop action
cleanup setup.cfg

- add cover-erase option
- add where=tests to limit nosetests discovery
remove .coverage in "make clean" action
fix bug in setting cachedir

- ensure global cachedir is set rather than just instantiating a local
  cachedir value that's thrown away in the module level fixtures
adjust warning in TemporaryDirectory

- ResourceWarning is only available in 3.2+, but we don't care
  about the specific warning in a test context, I think.  So
  changed this to a simple 'Warning' instance