Bug/url generate default params

Relates to https://github.com/BBC-News/alephant-broker/issues/28 created by @revett.

#### Problem

Would have been nice for users to not have to include `id` and `options` as arguments for the `#generate` method in their custom UrlGenerator class - however because the HTTP strategy **always** hands the method two arguments, there is no way around it.

#### Solution

Improve documentation.

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