Update Router.go signature to accept null for params

[harveysanders]

Resolves #104

This PR updates the signatures to Router.go, Router.path, and Router.url to allow passing null as the params argument. This makes code more clear when you want to call Router.go without any parameters, yet need to pass the qs argument.

Router.go('someRoute', null, { q: 'a+query' });

Previously you would need to pass undefined as the second argument.

Router.go('someRoute', undefined, { q: 'a+query' });

[dr-dimitru]

Superseded by #108