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
Do you mind removing this else? We can have just something like this:

```
if opts.Detch {
  // bla bla
  return nil
}
return c.hijack...
```