fix remote testing

All tests (except 1 that was failing anyway) pass on OS X now by running docker in a VM with shared folders configured.

```
DOCKER_HOST=tcp://localdocker:4243 TMPDIR=$(pwd) env/bin/python setup.py test
```

The failing test is `TestKillWithSignal`, which I haven't touched. It fails on master as well, running everything inside a clean ubuntu 14.04 vm. Even running a container such as the following does not work using docker 1.1.0 client/daemon:

```
ID=$(docker run -d busybox sleep 60)
docker kill -s SIGTERM $ID
docker ps $ID # it's still running
```

Temporary folders for a few tests are now properly cleaned up after the test is over instead of leaving them to accumulate for the OS to deal with.
Standard Python semantics on `dict.get` is that it returns `None` if the key doesn't exist.