Correctly check for @private tag (fixes #60)

A shortened version of the AST looks like this:

```json
{
  "comment": "... */",
  "description": "...",
  "params": [],
  "returns": [],
  "access": "private",
}
```

Even though my code uses `@private`, it still gets expanded to `access: "private"` in the AST.  This means that `tag.private` was always undefined, and `isInternal` was incorrectly returning false. This fixes that.