Support batch (multiple VM) requests

This PR allows a single API request to return multiple VM objects.  It
supports the following formats:

- POST <json> /vm/
  eg. 'curl -d '{"debian-7-i386":"3","debian-7-x86_64":"1"}' --url vmpooler/vm

- POST /vm/<pool>+<pool>+<pool>...
  eg. 'curl -d --url vmpooler/vm/debian-7-i386+debian-7-i386+debian-7-i386+debian-7-x86_64

Both commands listed above will return a JSON hash of 3 debian-7-i386 VMs and
1 debian-7-x86_64 VM:

  {
    "ok": true,
    "debian-7-i386": {
      "hostname": [
        "v49pwwk5yzg6oad",
        "ylghlgote5uso54",
        "wt0c4xovvulo7ge"
      ]
    },
    "debian-7-x86_64": {
      "hostname": "v3dkrulttp360fm"
    }
  }