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
Could this be part of an Exec interface? CreateExec could return an interface on which we can then invoke Start(). 
```
execObj, err := dockerClient.CreateExec(opts)
...
err := execObj.Start(opts)