Allow err params in callbacks and support objects in Client

[bobjflong]

If the callback has 2-arity, we assume the first param is err and use it when the API returns an error.list. This allows for idiomatic callback handling like:

client.users.list(function(err, d) { ... });

Additionally this pr adds support for objects in client construction:

var client = new Client({ appId: 'foo', appApiKey: 'bar' });

fixes #15

[josler]

🚢

[billinghamj]

@bobjflong Are there plans to deprecate the original behavior? Arity detection is generally considered bad practice and is likely to cause unexpected behavior.