Make graceful.Server a package-local type.
Ensure all error messages are not capitalized.
Included documented names in documentation.
Use a simplified loop.
Change requestId to requestID.
Remove return-only else branches.
Move type assertions into tests.
Change Url to URL.
Change Http to HTTP.
Use rt instead of m for the router receiver.
No need to re-document methods.
Minor cosmetic fixes
Fix up URLParams rename compilation errors

This fixes up the last few compilation errors caused by the UrlParams =>
URLParams rename.
Clarify /admin/ vs /admin in example app

I'm not particularly satisfied with the solution here. Maybe the answer
is "more middleware," but I'm not sure.
Also fix URLParams in tests
May as well rename this string too
Update documentation for URLParams rename
Fix Error => Errorf

This bugfix brought to you by `go vet`
Make graceful.Server public again

graceful.Server was made private in 05c2ca7e, but I think the increased
flexibility you get with being able to provide your own TLS options
(etc.) outweighs the API complexity of an additional type.