introduce minimal/recommended docker client version check

https://github.com/michaelsauter/crane/commit/d52a3f62fe9ba0504e7022d4d4313bcd6073db9d makes killing of containers slow on Docker < 1.2.0 as `docker rm -f` maps to a stop for these versions.

It's functional and I understand we shouldn't require anything else than > 1.0.0 unless we really should, but it makes restarting a lot of containers very slow, so I thought we could have a warning in that case. I generalized the checks so that crane starts which client is in the PATH at startup. This does not check anything about the corresponding server/engine version, as the client does that.
Of course, you're right. Just added an early return.