Ability to run containers on particular cpu cores

Added a -C argument to docker run, which sets lxc.cgroup.cpuset.cpus. That makes it possible to run containers on particular cpu cores, complementing the current -c argument. The following command will run the container on cores 0 and 1, with half a share each (using the convention that each core has 1024 shares total).

```
bin/docker run -i -t -c 512 -C 0 -C 1 761b29a3baab /bin/bash
```

I don't know of a satisfactory way to implement this on top of docker. It's very important to my application to have fine-grained control over cpu allocation.

Thanks!