Add support for specifying resources for a container

Hi!

This PR makes it possible to specify runtime constraints for a container, such as memory and cpu usage.
Currently it's not exposed in the cli, but you can set the parameters if you create a job from a json file.

### Example:
python-hello.json:
```json
{
  "image" : "google/python-hello:2.7",
  "resources" : {
    "cpuShares" : 512,
    "cpuset" : "0",
    "memory" : 104857600,
    "memorySwap" : 1024
  }
}
```

helios create -f python-hello.json python-hello:v1
```docker inspect``` on the running container will then return something like this:
```json
[
  {
    "Config": {
      "Cpuset": "0",
      "CpuShares": 512,
      "Memory": 104857600,
      "MemorySwap": 1024,
      "..."
    },
  }
]
```