Create gists

Adding support to create a gist.

One thing to note, is in order to get serialization correct I had to create an anonymous object and set Files to a JsonObject.  Otherwise, the Dictionary will serialize to an unexpected value that the API will reject.

```
{
  "description": "the description for this gist",
  "public": true,
  "files": {
    "file1.txt": {
      "content": "String file contents"
    }
  }
}
```