Simplify getting URI

[atymic]

Hey :)

First of all, thanks for the library, was going to write one very similar but found yours!.

Just a quick suggestion, currently i'm doing something like this in the code:

$url = $links->getRel('next') ? $links->getRel('next')->getUri() : null;

It's a bit tedious having to write a condition every time, as calling $links->getRel('next')->getUri() when there is no next link getRel returns null and causes an error.

Maybe we could add another method, such as: links->getRelUri('next') to try to fetch the url directly?

Let me know and I can create a PR.
I also noticed a couple of improvements that could be made, so can create a PR for those as well if you want :)