fix ListContainers which was failing with "executable true; not found"
support specifying location of temp folders

TestWithBinds was failing when shared folders are used because /tmp was
not shared from OS X to my VM. This fix allows the location of the temp
folders to be changed via:

    TMPDIR=$(pwd) env/bin/python setup.py test

This also properly cleans up temp folders which were sticking around
before.
allow docker client to connect to a remote host

to run the tests on a host without using the default unix socket, it's
now possible to specify:

    DOCKER_HOST=tcp://localdocker:4243 env/bin/python setup.py test
Merge pull request #261 from mmerickel/fix.remote-testing

fix remote testing