internal/router: add support for id resolving

We allow non-specific ids in API paths.
The details of the id resolution are kept
out of the router - that responsibility is
up to the API version implemention,
and will be implemented in a subsequent branch.
internal/router: add test
Merge pull request #27 from rogpeppe/011-resolve-id

internal/router: add support for id resolving

We allow non-specific ids in API paths.
The details of the id resolution are kept
out of the router - that responsibility is
up to the API version implemention,
and will be implemented in a subsequent branch.