Support for Docker exec

I haven't had a chance to look into tests yet but I thought it would be better to open up the PR to get some feedback before spending some time working on that :smile: 

Here's some code I'm using to test the functionality: https://gist.github.com/fgrehm/5c97dad1e61a1b187e77

Closes GH-160
I thought of adding that link but other links to docs on the codebase point to https://docs.docker.com/reference/api/docker_remote_api_v1.14 instead of the git source. @fsouza what do you think?