Discrepancy in documentation for GitHub.search()

In [lib/GitHub.js](https://github.com/github-tools/github/blob/22b889cd48cd281812b020d85f8ea502af69ddfd/lib/GitHub.js#L97), the argument for `GitHub.search()` is a string, `query`.

This gets passed to the `Search` constructor, which takes an object [as per here](https://github.com/github-tools/github/blob/master/lib/Search.js#L18). I think I tried this awhile back and it was throwing errors as a result.

Compare the GitHub.search() argument documented in lib/GitHub.js with the object expected by the Search constructor in lib/Search.js. Confirm the intended interface and update the documentation so it matches, then verify the search entry point no longer presents an invalid argument shape.

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