Controls with role="presentation" do not need labels.

Adding role="presentation" is an explicit declaration by the developer to hide the elements native semantics from the accessibility API, and thus should be ignored.

http://www.w3.org/TR/wai-aria/roles#presentation

[Closes #23]
@alice what do you think about splitting this out into a helper, like `axs.utils.hasRolePresentation(element)`?