Make private methods private

[roback]

Something like this:

Client.prototype.query = function() {
    return new Query(this);
};

// ...

// This is "private"
function apiKeyMissing() {
    throw new TwinglyAuthError('No API key has been provided.');
};

module.exports = Client;

This is the only way I can think of right now, but there may be other better solutions that I haven't heard of 😄.