Provide snapshot content as JSON - #152

```json
$ http --json http://localhost:4000/lts-5.1
{
  "snapshot": {
    "ghc": "7.10.3",
    "created": "2016-01-30",
    "name": "lts-5.1"
  },
  "packages": [
    {
      "isCore": false,
      "name": "abstract-deque",
      "version": "0.3",
      "synopsis": "Abstract, parameterized interface to mutable Deques."
    },
    {
      "isCore": false,
      "name": "abstract-par",
      "version": "0.3.3",
      "synopsis": "Type classes generalizing the functionality of the 'monad-par' library."
    },
    ...
  ]
}

```