Promotes Variant to mapbox:: Namespace — Closes #104

[daniel-j-h]

For https://github.com/mapbox/variant/issues/104 - cc @artemp @jfirebaugh

In a future version we want to remove the mapbox::util:: namespace. Until then we provide both adding a deprecation warning to the readme.

Implementation detail: we make the util namespace an inline namespace - otherwise users could notice us changing the specific namespace in which our templates were declared in.

The downside here is we can't add a deprecation pragma since then the mapbox:: namespace would contain it, too.